Teknik Pembakaran English Version

DEPARTEMEN TEKNIK KIMIA UNIVERSITAS INDONESIA

COMBUSTION ENGINEERING

SUBJECT :

COMBUSTION

ENGINEERING

Oleh : Dr.Ing Donni Adinata, ST., M. Eng. Sc

1

SYLLABUS

Introduction

Basics of Combustion

Combustion System

Chemical Kinetics of Combustion

Combustion Chemistry

Laminar Premix Flame

Turbulent Premix Flame

Laminar (Diffusion) Non-Premix Flame

Turbulent Non-Premix Flame

2

Combustion – Fundamental and Application, J. Warnatz,

dkk, 1999

Combustion, I. Glassman, 1996

Simulating Combustion, G.P. Merker, dkk, 2004

Industrial Combustion Pollutants and Control, C.E.

Baukal Jr., 2004

Combustion Physics, C.K. Law, 2006

Combustion – Fundamentals and Technology of

Combustion, El-Mahallawi, 2002

Combustion Theory, Williams, 1985

REFERENCE

3

PENDAHULUAN

Dr. rer. nat. Ir. Yuswan Muharam, M.T.

4



SUBJECT :

INTRODUCTION

Oleh : Dr.Ing Donni Adinata, ST., M. Eng. Sc

WHAT IS COMBUSTION?

Main source of driving energy for the technology

community (~85% of world energy consumption

Main source of air pollution;

Utilization

– Driving force of aircraft and spacecraft, power plant,

heating, transportation, and material processing

5

APPLICATION OF COMBUSTION

Gas turbine and jet engines

Rocket thrust

Piston engine

Weapons and explosives

Furnace dan boiler

Synthesis of materials with flame (fullerene, nanomaterial)

Chemical processing (black carbon production)

Material formation

Fire hazards and safety

6

Technique

– Reaction of fuel with oxidant

– Exotermic

– Self-sustaining,

– Through some chemical and physical events

– Form water and carbondioxide (the most stable

reaction product)

DEFINITION

7

CLASSIFICATION

Conventional Combustion

– Oxidation of fuel that is accompanied by flame or high temperature

Non-conventional Combustion

– Oxidation of fuel that is not accompanied by flame or high temperature

8

CLASSIFICATION

Subsonic combustion or deflagrasi

– Occurs in daily life

– Propagation speed of combustion reaction wave is lower than speed of sound

Supersonic combustion or detonation

– Propagation speed of combustion reaction wave is higher than speed of sound

9

COMBUSTION PROCESS

Complex interaction of

– Physical process

• Fluid dynamics

• Heat transfer

• Mass transfer

– Chemical process

• thermodynamics

• Chemical kinetics

– Practical applications involving other disciplines :

aerodynamics, fuel technology, and machine engineering.

10

THERMODYNAMICS

Stoichiometry

Properties of gases and its mixture

Heat of formation

Heat of reaction

Equilibrium

Adiabatic flame temperature

11

TRANSPORT PHENOMENA

Heat Transfer

• Conduction

• Convection

• Radiation

Mass Transfer

• Total,

• Species

Momentum Transfer

• Laminar Flow

• Turbulent Flow

• Inertia and viscosity effects

• Combustion aerodynamics

12

CHEMICAL KINETICS

Application of thermodynamics to the reaction system that produces :

– Chemical composition of combustion products

– Maximum temperature (adiabatic flame temperature)

However, thermodynamics alone is unable to inform that the system will

reach the equilibrium or not. If the timescale of chemical reactions

involved in combustion process is proportional to the timescale of

physical process (such as diffusion, fluid flow) that occur

simultaneously, then the system may never reach equilibrium.

Therefore, we need rate of chemical reaction in combustion

13

Fuel

– Fossil-HC based

• Natural gas (methane, ethane, and propane)

• Petroleum products (gasoline, diesel, jet fuel, oil fuel)

• Coal and its products (synthetic gas and liquid)

Oxidant

– Oxygen from the air

• Hydrogen and oxygen are used to drive rocket and on the fuel

cell

– Fuel and oxidant are part of the same molecule

• Explosives (such as TNT) and solid propellants

COMPONENTS OF COMBUSTION

14

HISTORY OF FOSSIL FUEL

Before 1900s: wood,

Early 1900s: coal,

1900s:

– Petroleum products (almost all transportation)

– Coal (power generation)

End of 1900s : natural gas (heating, cooking, power

generation and transportation)

15

TOTAL PRIMARY ENERGY SUPPLY IN MTOE

Source: International Energy Agency

16

1 Mtoe = 16,3 GJ

Other : geothermal, solar, wind, etc.

OUTLOOK OF TOTAL PRIMARY ENERGY SUPPLY IN a MTOE

17

EMISSION

Influencing variable :

– Type and composition of fuel,

– Ratio of fuel and oxygen,

– Design of combustion system,

– Operating condition (initial temperature and

pressure),

– Additive

18

CLASSIFICATION OF EMISSION

Not a pollutant

– CO2 and H2O.

Pollutant :

– Unburnt fuel;

– Nitrogen oxide(NO, NO2, and N2O, or NOx),

– Sulfur oxides (SO2 and SO3, or SOx),

– Product of imperfect combustion (PIC),

• CO,

• Asiri organic compound (VOC), such as ethane, ethylene, propane,

acetylene and solvent, oxygenate (aldehyde, ketone, alcohol, peroxide),

• Aromatic,

• Polycyclic aromatic carbon (PAH),

• Particulate (solid carbon or soot)

– Halogenated compound,

– Metal

19


20


21

EMISSION

CO2

– Source : all combustion ;

– Hazard to health

• > 5000 ppm > 2 – 8 hours,

– Accelerating the pace

• Levels in atmosphere increased from 280 ppm (pre-industrial

times) to > 350 ppm (1990);

– Greenhouse gases

• Along with other greenhouse gases (exampe : methane),

CO2 absorbs infrared radiation which is emitted by earth,

thus energy in the earth increases and atmospheric

temperature rises.

– Become a global issue, after Kyoto Protocol in 1997.

22

World CO2 emissions by fuel source (in 106 t)

23

World CO2 Emissions by Region (in 106 t)

24

EMISSION

CO

– Source : motor vehicles, industrial processes

– Health hazards :

• 9 ppm (10 mg/m3) > 8 hours,

• 35 ppm (40 mg/m3) > 1 hour,

• Not more than once a year (for both)

– Absorbed by the lungs;

– Weakening the physical and mental;

– Affect embryo development.

25

EMISSION

NOx

– Source : motor vehicles; heat and electricity geneartor; nitric

acid; explosives; fertilizer factory.

– Hazard to health :

• NO2: 0,053 ppm (100 µg/m3) > a year;

– Reacts with HC and ultraviolet to form oxidant fotochemical

haze,

– Respiratory problems and heart disease.

26

EMISSION

• SOx

– Source : power plant uses thermal from oil and coal containing

sulfur, sulfuric acid plant,

– Hazard to health :

• SO2:

– 0,03 ppm (80 µg/m3) > a year,

– 0,14 ppm (365 µg/m3) > 24 hours,no more than once a

year,

– 0,5 ppm (1300 µg/m3) > 3 hours.

– Causes middle level irritation

– Main cause of acid rain.

27

EMISSION

VOC includes ethane, ethylene, acetylene, propane,

butane, pentane, aldehydes, ketones, solvents.

– Source : motor vehicles; evaporation of solvent; industrial

processes; disposal of solid waste; burning fuel; oil refineries;

fuel pump station; cleaning clothes; printing; paint.

– React with Nox and ultraviolet to form photochemical oxidants.

– Acute exposure causes irritation of the eyes, nose and throat;

chronic exposure causes cancer.

28

EMISSION

29

CONVENSIONAL COMBUSTION CONTROL

Design of reactor or combustor that has resistant to high

temperatures generated

– For example :

o Combustion engine cylinder of gasoline-fueled vehicles,

o Alloy tube which has resistant in high temperature on energy

generating system such as gas turbine and furnace.

Design of combustion process

o Operating condition,

o Fuel composition,

o Ratio of fuel and air

30

NON-CONVENSIONAL COMBUSTION CONTROL

T is lower than flame temperature

For example :

– H2-air fuel cell at 80 °C, while the hydrogen-air flame

at 2000 °C.

– Catalytic combustor operates at 800oC and fuel is

processed at wide temperature range.

31



SUBJECT :

COMBUSTION

FUNDAMENTALS

Oleh : Dr.-Ing Donni Adinata, ST., M. Eng. Sc

32

INTRODUCTION

Three fundamental components :

Fuel + Oxidizer + Diluents Combustion Products

33

FUEL

Gaseous Fuels

Liquid Fuels

Solid Fuels

34

GASEOUS FUELS

Predominant fuel source in most of application.

Contain multiple components such as methane, hydrogen,

propane, nitrogen and carbon dioxide.

Sometimes referred to as refinery fuel gases.

The easiest to control because no vaporization is required.

Simpler to control to minimize pollution emissions because

they are more easily staged.

35

GASEOUS FUELS

36

LIQUID FUELS Used in some limited applications.

Waste liquid fuels are used in incineration processes.

Challenges of using oils

o Vaporizing the liquid into small enough droplets to burn completely.

• Improper atomization produces high unburned hydrocarbon

emissions and reduces fuel efficiency

• Steam and compressed air are commonly used to atomize liquid

fuels

• The atomization requirements often reduce the options for

modifying the burner design to reduce pollutant emissions.

o Containing impurities like nitrogen and sulfur

In the case of fuel-bound nitrogen, so-called fuel NOx emissions

increase

In the case of sulfur, all of the sulfur in a liquid fuel converts to

SOx emissions

37

Liquid fuels

38

LIQUID FUELS

39

LIQUID FUELS

The advantages of liquid fuel

o The flames much more luminous

• Caused by the high solid carbon content which produces

infrared radiation when heated.

• Enhance the radiation heat transfer from the flame to the

material being processed.

• Indirectly reduce pollution emissions because the higher heat

transfer can improve the thermal efficiency which means that

less fuel needs to be burned.

40

SOLID FUELS

Not commonly used in most industrial combustion

applications.

The most common solid fuels

o Coal in power generation

o Coke in some primary metals production processes.

o Sludge (pseudosolid fuel) in incinerators.

Contain

o Impurities such as nitrogen and sulfur

o Hazardous chemicals

41

OXIDIZER

Air (oxidant) air/fuel combustion

79% nitrogen diluent

21% oxygen oxidizer

Oxygen-enhanced combustion (OEC)

Air blended with pure O2

High purity O2 oxy/fuel combustion

42

AIR/FUEL BURNER

43

AIR ENRICHED WITH O2 BURNER

44

O2 LANCING BURNER

45

OXY/FUEL

46

AIR-OXY/FUEL

47

INTERNAL COMBUSTION ENGINE

The combustion of fuel and an oxidizer

(typically air) occurs in a confined space called

a combustion chamber.

This exothermic reaction creates gases at high

temperature and pressure, which are permitted

to expand.

Useful work is performed by the expanding hot

gases acting directly to cause movement of

solid parts of the engine, by acting on pistons,

rotors, or even by pressing on and moving the

entire engine itself.

48

INTERNAL COMBUSTION ENGINE

49

DILUENT

To reduce and moderate the flame temperatures that reduce NOx

emissions.

To change the heat-transfer distribution from the flame. The flame

can be stretched to make the flame radiation more uniform by

dilution.

To increase the convection heat transfer in the furnace (by adding

to the flame).

Example:

o Products of combustion that are recycled back toward or into the burner.

o Water, steam, and gases like nitrogen or carbon dioxide.

50

RECIRCULATION

Furnace gas recirculation (FuGR)

– The combustion products are drawn back into the

flame inside the furnace.

Flue gas recirculation (FlGR)

– The combustion products are drawn back into the

flame outside the furnace.

51

RECIRCULATION

• For improved thermal efficiency

– Enhanced convective heat transfer inside the combustor due to

the improved fluid flow and the increased residence time of the

hot gases in the combustor.

• Reduced NOx emissions.

– Reduces the peak flame temperatures in the combustion zone

that are the primary source of thermal NOx emissions

52



SUBJECT :

COMBUSTION CONTROL


53

COMBUSTION CONTROL

Performance parameters of the combustion process:

- Energy;

- Flame temperature;

- Pollutants;

- Otoignisi;

- Flame propagation speed;

Control parameters in combustion process:

Reactor design (the engine) that resists with high temperatures,

- Cylinder design of gasoline engine and diesel.

- Alloy tube power systems (gas turbine and furnace).

Design of the combustion process

- Operating conditions, fuel composition, the ratio of fuel and air.

54

COMBUSTION CONTROL

Necessary to control the combustion

o Chemistry of combustion

- Chemical reaction;

- Chemical kinetics;

- Thermodynamics.

o Fluid dynamics

- Mass balance,

- Energy balance,

- Motion equations,

- Transport parameters (diffusion, turbulence, dispersion),

- Material properties (viscosity, density, thermal conductivity, heat

capacity).

55

FLAME TYPE

Premix Flame

o Laminar

o Turbulen

Non- Premix Flame (Diffusion)

o Laminar

o Turbulent

Parsial Premix Flame

o Laminar

o Turbulent

56

LAMINAR (TURBULENT) PREMIX FLAME

Fuel (gas) and oxidizer are mixed homogeneously

before burning

Laminar flow (turbulent)

Premix turbulent flame:

o Combustion in gasoline engines

o Combustion in gas turbines

57

COMBUSTION IN GASOLINE ENGINE

58

LAMINAR (TURBULENT) PREMIX LAMINAR

Stoichiometry:

o Premix flame is called stoichiometric if the reactant

mixture containing oxidizer in the appropriate

quantities to react with the fuel (burned) is perfect.

o If the fuel is in excess: fuel rich system.

o If the oxygen is in excess fuel poor system

o Standard air composition:

59


Stoichiometry:

o (A / F) Stoic

Mass ratio of air-fuel

(air mass) / (fuel mass)

o (A / F) Stoic = [5 (32 +3.762 * 28)] / (44) = 15.6

o Φ = equivalence ratio of fuel

(A / F) Stoic / (A / F) actual

60


stoichiometry:

o Φ = 1: combustion stiochiometry

o Φ <1: thin mixture , lean combustion

o Φ> 1: rich mixture , rich combustion

o The European and the Japanese Convention use air

equivalence ratio, λ

λ = 1 / Φ

61

LAMINAR (TURBULENT) NON-PREMIX LAMINAR

Fuel (gas) and oxidizer are mixed during the combustion

process

Examples of laminar non-premix flame:

- Wax flame

Examples of turbulent non premix flame:

- Hydrogen rocket engine

- diesel engines

62

LAMINAR (TURBULENT) NON-PREMIX LAMINAR

Wax Flame

63

EXAMPLE OF COMBUSTION SYSTEM

64



SUBJECT :

COMBUSTION

STOICHIOMETRY


65

COMBUSTION STOICHIOMETRY

To calculate how much air is used to oxidize the fuel

completely into CO2, H2O, N2 and SO2.

Complete combustion of CH4 in the air:

.

.

.

.

.

66


Volume ratio of the stoichiometric air-methane, AFRv,:

Mass ratio of the stoichiometric air-methane, AFRm,:

67

NON-STOICHIMETRY MIXTURE

Poor fuel mixture:

- Lack of fuel than the stoichiometric ratio;

- Combustion may be perfect;

- Excess oxygen in the product.

Rich fuel mixture:

- Excess fuel than the stoichiometric ratio;

- Combustion may not be complete;

- Intermediate in the product.

68


Equivalence ratio, :

– < 1 poor fuel;

– > 1 rich fuel.

69


Air-fuel relative ratio, 1 /:

– AFRactual/AFRstoichiometry;

– Also called an equivalence ratio of oxidizing agent

Percent theoretical air, 100 / ;

Percent excess air, EA:

70


a, a1, a2 …. = coefficient;

= equivalence ratio;

Other species can be added on the right side;

71


If = 1, so complete reaction

become :

72


If <1 and the complete reaction, a2 = a4 = 0;

from the atom balance sheet of C, H, and O :

– a1=x, a3=y/2, and a5=a(1-)/.

– (1-)/ is called "excess air".

73


If > 1, the composition of the final product should be

calculated using the equilibrium (there is CO and H2 , so

the amount of unknown variables is more than

equations).

74

EXAMPLE

Hydrocarbon fuels the composition are 84.1% mass C

and 15.9% mass of H has a molecular weight of 114.15.

Calculate the moles amount of air required for

stoichiometric combustion moles amount of product

produced per mole of fuel. Calculate AFR stoichiometric!

75

ANSWER

Assume fuel composition is CaHb.

Molecular Weight = 114.15 = 12.011a + 1.008b

Gravimetric analysis:

a = 8 ; b = 18

fuel is octane

76

ANSWER

77

Stoichiometric Combustion

1 mol of fuel 59,66 mol air

64,16 mol product.

AFRstoichiometric = 59,66

78

ANSWER



SUBJECT :

FIRST LAW OF

THERMODYNAMICS


79

ENERGY BALANCE AND ENTHALPY

80


Constant Volume, TR = TP = T’

becomes

Heat reaction at constant volume

in T’ (per mol)

81


Constant Pressure, TR = TP = T’

becomes

Heat reaction at constant

pressure at T’ (per mole) 82

Relation of (H)P,T’ and (U)V,T’ :


83


Combustion product is H2O in vapor and liquid phase

84


Fuel in gas and liquid phase

85

ENTHALPY OF FORMATION

Formation enthalpy of a compound:

o The increase of enthalpy is associated with formation reaction of

one mole compound from its elements, where each elements is at its

standard thermodynamic state at a certain temperature.

Default state:

o At 1 atm pressure and certain temperature.

Datum state:

o All other thermodynamic state were referred to this state;

o Usually at 298.15 K (25oC) and 1 atm;

o Enthalpy of the elements in its reference state at datum temperature

is zero ;

o Element reference state is a stable standard state :

Oxygen at 298.15 K, the reference state is gaseous O2.

86

ENTHALPY OF FORMATION

Enthalpy of products at the standard state is relative to

the datum enthalpy:

Enthalpy of reactants at the standard state is relative to

the datum enthalphy:

The increase of enthalpy:

87

EXAMPLE

Calculate the enthalpy of products and reactants and the

increase of energy in the stoichiometric mixture reaction

of methane and oxygen at 298.15 K!

88

ANSWER

89

STANDARD ENTHALPY

Sensibel

Enthalpy

Standard

Enthalpy

Temperature function of enthalpy :

Consists of two sets of coefficients (NASA program):

1. 300 K – 1000 K

2. 1000 K – 5000 K

90

STANDARD ENTHALPY

91

HEAT VALUE

If the fuel composition is not known, the enthalpy of the

reactants can not be calculated from its enthalpy of

formation;

Heat value of fuel:

a. Reaction heat at constant pressure (volume) at standard

temperature (usually 25 ° C) for complete combustion of fuel;

b. Measured in the calorimeter;

c. J / kg or J / kmol of fuel.

92

HEAT VALUE

High heating value (gross heating value):

o All H2O condensed into a liquid phase;

Low heating value (net heating value)

o All H2O is in the vapor phase.

93

ADIABATIC COMBUSTION

Adiabatic combustion in constant volume :

The data of internal energy or enthalpy is given relative

to its value at the reference temperature T0, U(T) - U(T0)

94


Adiabatic combustion in constant pressure :

95


Adiabatic flame temperature

96



SUBJECT :

CHEMICAL KINETICS


97

CHEMICAL REACTION

The different species molecules collide, producing one or more

new molecules;

Atoms of reactant molecules are distributed back to the new

molecules;

Reactant molecules must have enough kinetic energy to break

chemical bonds during the collision and new bonds are formed;

Energy content of the collision products differs from the energy

content of molecules that collide Basis of release or absorption

of heat in chemical reactions.

98

GLOBAL REACTION (OVERALL)

Combustion of 1 mole of CH4 and 2 moles of O2 to produce 1 mole

of CO2 and 2 moles of H2O (complete reaction).

Number of reactant molecules which collide to produce products are

not the same as indicated by the global reaction.

The molecules that collide may not

have enough kinetic energy to reach so many replication

of the bonds required by the global equation.

99

ELEMENTARY REACTION

Reactions that occur at the molecular level which

are described in accordance with the equation

of chemical reaction.

shows that 2 moles of H2 react with 1 mole of O2 to

produce 1 mole of H2O NOT TRUE!

100

ELEMENTARY REACTION

Reality: a sequential process that

involves several intermediate species:

First reaction termination of the H-H and O-O bonds, the

formation of two O-H bonds and an atomic H.

101

ELEMENTARY REACTION

Radicals or free radicals or reactive species : reactive

molecules or atoms that have unpaired electrons.

Complete description for burning process of H2 with O2

more than 20 elementary reactions.

102

CHAIN REACTION

Combustion of hydrocarbons

Tens to hundreds of species and radicals;

Hundreds to thousands of elementary reactions that arrange the

overall reaction reaction mechanism or detail chemical

mechanisms;

The process of producing products, initiate other similar processes

automatic continuity.

103

CHAIN REACTION

2.50: chain initiation (reactive intermediate is formed through the

action of heat or something like O2 molecules).

2:51: chain branching (making more radical).

2:52: chain propagation ( change of radical identity, but the amount

are still same).

2:53: chain termination (radicals is consumed and the chain ends).

104

TYPE OF ELEMENTARY REACTION

Based on the number of reactant molecules :

o Elementary reactions

overall order (a1 + a2 + a3 + ...),

a1 is order of the reactants R1 ,etc.

o Overall order is called molecularity.

105

TYPE OF ELEMENTARY REACTION

Based on its molecularity:

- unimolecular reaction

- bimolecular reaction

- trimolecular reaction

106

REACTION RATE COEFFICIENT DEPENDENCE ON TEMPERATURE

Arrhenius law:

o A’ = pre-exponential factor;

o Ea = activation energy;

o exp(- Ea/RT): the proportion of collisions that occur

between molecules that have kinetic energy greater

than Ea.

107

REACTION RATE COEFFICIENT DEPENDENCE ON TEMPERATURE

Binary reactions:

o Arrhenius behavior in the middle temperature range;

o Dependence of the rate coefficient on temperature is in

the exponent.

Low activation energy reaction with a wide temperature

range:

o Behavior of "non-Arrhenius" modified Arrhenius

108

REACTION RATE COEFFICIENT DEPENDENCE ON PRESSURE

Decomposition reaction (unimolecular)

A B + C

and recombination (bimolecular)

A + B C + D

o Rate depends on the pressure;

o The reaction is not elementary (consisting of a number

of reactions);

o Lindemann Model: study on the rate coefficient dependence on

pressure

109


Unimolecular decomposition :

o Need energy to break its bonds;

o Energy is transferred to the molecule through collisions with

molecules M (to stimulate the molecule vibrations);

o Excited molecule decomposes into a product, or is deactivated

through the second collision, depending on the strength of stimulation.

110


reaction rate

** *a a u

d Ak A M k A M k A

dt

111


Assumption: the concentration of A * in is quasi-steady

state

** *a a u

d Ak A M k A M k A

dt

112


Extreme condition

o P <<<

[M] <<<

Border stage : the activation

o P >>>

[M] >>>

Border stage: the decomposition of A *.

113

Second order

First order


At P <<<, k tends toward k so that k is

almost independent of P.

At >>> P, k ~ linear dependence.

114

REACTION MECHANISM

Reactants which are consumed and produced arises

from the sum of the contribution of each elementary

reaction.

Example:

The rate of CH3 formation and CH4 consumption

115

REACTION MECHANISM

Mechanism consists of R elementary reactions from

S species,

where r = 1, 2, ..., R,

= stoichiometric coefficients of reactants and products.

Formation rate of species i:

where i = 1, 2, ..., S.

116

ANALYSIS OF REACTION MECHANISM

Detailed reaction mechanism of hydrocarbon combustion

consists of hundreds of elementary reaction. However,

some of them are not important so it can be eliminated.

Analytical methods are needed to eliminate unimportant

reactions, among them:

Sensitivity analysis: identifying the rate limiting reaction steps.

Reaction flow analysis: identifying the characteristics of the

reaction pathway.

Information obtained from these two methods can

be used to dispose unimportant reactions to make

mechanism becomes simple or reduced.

117

SENSITIVITY ANALYSIS

Rate law of a reaction mechanism consisting of R

reaction among S species can be written as a system

of first-order differential equations,

t: independent variable, ci: the dependent variable, kr : the

system parameters.

Solution of ordinary differential equations system

depends on initial conditions and system parameters.

118


On a number of elementary reactions, changes of kr almost

have no effect on system output

- The reaction is eliminated;

- kr does not need to be accurate.

On a number of other elementary reactions, changes in kr is

very influential on the system output

- kr needs to be accurate;

- Rate limiting or determining stage.

Dependence of ci solution on kr is called sensitivity.

119

ANALYSIS OF REACTION FLOW

Reaction flow analysis calculate the contribution

percentage of each reaction to the establishment or use

of chemical species.

120



SUBJECT :

FLAME TEMPERATURE


121

ADIABATIC FLAME TEMPERATURE

Two-stage analysis,

a. First: the reaction at 298.15 K: heat is released; amount

of heat is calculated based on the amount of fuel and the heat

of combustion;

b. Second: the heat resulted form first stage is used to raise the

temperature of the product from 298.15 K to the final

temperature.

122

ADIABATIC FLAME TEMPERATURE

From the amount of enthalpy , we calculate the temperature in which the

rise of total enthalpy is equal to the heat released by combustion. We can

obtain the final product temperature (calculate Tf in such a way so that

the First Law of Thermodynamics are fulfilled:

Tf is reduced if :

o There is heat loss from the system,

o Tin is less than 298.15 K,

o Phase change, for example some of the heat used for evaporation

1 1reak prod

N N

i ii in i f

i i

n h T n h T

123

EXAMPLE

Calculate the adiabatic flame temperature at constant

pressure for propane which is burned

with air in composition of 21% O2 and 79% N2 (by

volume) at =1. Assume complete combustion

occurs, Pin = Pf = 1 atm and Tin = 60 F.

124

DATA

125

ANSWER

On the stoichiometric and complete combustion conditions:

For propane:

C3H8 + 5(O2 + (0,79/0,21)N2) 3CO2 + 4H2O + (5*0,79/0,21)N2

Composition of air : O2 = 21% volume ; N2 = 79% volume

Initial temperature = 60 F = 520 R

Initial pressure = 1 atm;

R = 0.7302 3/lbmol/R

126

ANSWER

Assume initial volume of the mixture = 1 ft3

Mole of mixture (propane, oxygen and nitrogen) in the vessel

n = (PV) / (RT)

n = (1 atm)(1 ft3) / (0.7302 atm.ft3/lbmol/R

(520 R)

n = 0.00263 lbmol.

Stoichiometric fuel = (1/5) x volume of O2

= (1/5) x 21 = 4.2

127

ANSWER

For gas, volume fraction = mole fraction, then

o Mole fraction of propane = (4.2) / (4.2 +21 +79) = 0.04031

o Mole fraction of oxygen = (21) / (4.2 +21 +79) = 0.20154

o Mole fraction of nitrogen (79) / (4.2 +21 +79) = 0.75816

reactant

o nC3H8 = (0,04031)(0,00263) = 0,00011 lbmol

mC3H8 = (0,00011 lbmol)(44 lb/lbmol) = 0,00484 lb

o nO2 = (0,20154)(0,00263) = 0,00053 lbmol

o nN2 = (0,75816)(0,00263) = 0,00199 lbmol

128

ANSWER

Product

o nCO2 = (3)(0,00011) = 0,00033 lbmol

mCO2 = (0,00033 lbmol) (44 lb/lbmol) = 0,01452 lb

o nH2O = (8)(0,00011)/2 = 0,00044 lbmol

mH2O = (0,00044 lbmol)(18 lb/lbmol) = 0,00792 lb

o nN2 = 0,00199 lbmol

mN2 = (0,00199 lbmol)(28 lb/lbmol) = 0,05572 lb

129

ANSWER

Solved to obtain T;

T = 3556 R = 3096 F = 1702 oC

130

EXAMPLE

If propane is replaced by methane with the same number

of moles (0.00011 lb mol) and burned with air

stoichiometrically, how much is the adiabatic flame

temperature?

131

ANSWER

CH4 + 2(O2 + (0,79/0,21)N2) CO2 + 2H2O + (2*0,79/0,21)N2

Reactant

o CH4 = 0,00011 lbmol = 0,00176 lb

o O2 = 2(0,00011) = 0,00022 lbmol

o N2 = (0,75816)(0,00263) = 0,00083 lbmol

Product

o CO2 = (1)(0,00011) = 0,00011 lbmol = 0,00484 lb

o H2O = (4)(0,00011)/2 = 0,00022 lbmol = 0,00396 lb

o N2 = 0,00083 lbmol = 0,02324 lb

132

ANSWER

Solved to obtain T

T = 3584 R = 3124 F = 1717 ° C

What if the mole number of fuel is different?

133

HOMEWORK

What is the adiabatic flame temperature of

– an ethylene (ethene, C2H4)-air mixture that contains exactly and twice as much

oxygen as is necessary (K-1)

– a n-heptane-air mixture that contains exactly and twice as much oxygen as is

necessary (K-2)

– a isooctane-air mixture that contains exactly and twice as much oxygen as is

necessary (K-3)

– a n-octane-air mixture that contains exactly and twice as much oxygen as is

necessary (K-4)

to burn the fuel completely to CO2 and H2O? Initial mixture temperature

is 298 K and combustion takes place at constant pressure of 1 atm.

Assume complete combustion and no dissociation

134

HEAT OF NON-ADIABATIC COMBUSTION

135

Calculate the amount of heat released by combustion

of 0.00484 lb propane with a stoichiometric air at 60 F

and constant pressure of 1 atm, and if the temperature of

exit flue gas is 1400F!

EXERCISE

136

Enthalpy curve of water, CO, CO2 and SO2 at T > 60 F

137

JAWAB

Enthalpy curve of hydrogen, air and oxygen at T> 60 F

138

JAWAB

Enthalpy curve of carbon dioxide

139

JAWAB

Enthalpy curve of nitrogen

140

Enthalpy curve of water

141

PRESSURE CONVERSION

142

ANSWER

Mass

– mH2O =

– mCO2 =

– mN2 =

– mC3H8 =

At 1400 F

– hH2O =

– hCO2 =

– hN2 =

– Q =

143

Heat generated from the

combustion of refinery gas in

T0 = 60 F and the exit temperature

Tf exit.

144

EFFICIENCY OF COMBUSTION

Energy fuel which is

supplied to the control volume

145

CHEMICAL EQUILIBRIUM

Second Law of Thermodynamic

The equilibrium constant at constant pressure

p0 = standard pressure (usually 1 atm)

Reactant: vi (-); product: vi (+)

146


Effect of temperature on the equilibrium:

2

00

T

H

T

G

Tp

SUBSTITUTION

2

0lnln

RT

H

dT

Kd

T

K p

p

p

dTRT

HKd p 2

0

ln

147



- Kirchhoff equation

- In a specific scale

- Substitution of the specific heat:

- The result:

dTchd pˆˆ0

RdTTaTaTaTaahd 4

5

3

4

2

321

0ˆ

148



o Integration

o Substitution of the specific enthalpy

o integration

RTa

Ta

Ta

Ta

Tahh

55443322

1

0

0

0

5432ˆˆ

dTRT

RTa

Ta

Ta

Ta

Tah

dTRT

hKd p 2

554433221

0

0

2

05432

ˆˆ

ln

C201262

lnln 45342321

0

0

T

aT

aT

aT

aTa

RT

hKp

149


Effect of pressure on the equilibrium:

= 0: Changes of pressure do not affect the composition;

> 0: mole fraction of dissociation products decreases with increase of

pressure;

<0: mole fraction of reactant decreases with increase of pressure;

150


The equilibrium constant Kc

Relation of Kc and Kp relations (p0 = 1 atm)

151

HOMEWORK

Develop a general equation for Kp as a function of

temperature and calculate Kp at temperature of 1000 °

C for following water-gas reaction :

152



SUBJECT :

SIMPLE

THERMOCHEMICAL MODEL


153

SIMPLE THERMOCHEMICAL MODEL

Known : mixture of fuel and air at P and T.

Assumptions:

o Combustion chemistry:

Fuel + v Air Product,

o Total Stoichiometric:

1 Kmol fuel requires v kmol of air;

1 kg fuel needs S kg of air;

S = v MWair/MWfuel.

154

Global kinetics:

o yfu = mass fraction of fuel;

o yox = mass fraction of oxygen.

155


Fuel mass balance:

Energy balance:

Equations of motion:

156


These three equations are solved using the boundary

conditions;

Result :

o Concentration profile (conversion) of fuel;

o Oxygen concentration profile (with a stoichiometric relationship);

o Transient temperature profile along the combustion chamber;

o Flame propagation velocity profile;

157


COMPLETE THERMOCHEMICAL MODEL

Using reaction mechanism;

The rate of formation reaction / consumption of all species;

Mass balance of all species (hundreds of species, hundreds of

equations);

Energy balance (an equation);

Motion equations (one equation);

Hundreds of differential equations with boundary conditions solved

using the program

o Homrea

o Chemkin

o Mixfla

158

RESULT

o Concentration profile (conversion) of fuel;

o Concentration profile of O2, CO2, CO, H2O, H2, formaldehyde,

acetaldehyde, propionaldehida, methane, ethane, butane,

propane, ethylene, acetylene, butene, methanol, ethanol,

propanol, ketones, etc.

o Delay time of ignition;

o Flame propagation velocity profile.

159


Setting the operating conditions (P and T), equivalence

ratio and fuel composition:

o Minimize pollutants,

o Set the flame temperature;

o Set the speed of flame propagation;

o Know the delay time of ignition.

160




SUBJECT :

FLAME


161

FLAME

Reaction zone that moves relative to the gas that

sustains it.

Rapid exothermic reaction

Accompanied with light emission

162

FLAME

Premixed flame :

o Reactants are mixed before approaching zone of flame.

o Mixture of initial fuel and oxidant is between particular

composition limit (flammability limit)

Diffusion flame :

o Mixing fuel and oxidant, and combustion occurs in inter-phase

163

FLAME

premixed diffusion

164

PREMIXED FLAME

Has adiabatic flame temperature and flame rate (flat

front flame rate that is normal to the flame surface and

relative to non-burning reactants)

Occurs if the initial mixture is between the particular

composititon limit (flammability limit).

165

FLAMMABILITY LIMIT

Lower flammability limit (lean limit) :

o Flash point is reached when the fuel gas in small amounts are

added little by little into the air ;

Upper flammability limit (rich limit) :

o Point that is reached if fuel is added again when the mixture is

no longer cause flame.

Flammablility range becomes wider if :

o Temperature of mixture increases

o Mixture pressure increases above atmospheric pressure

The broadening occurs at the upper limit.

166

167

FLAMMABILITY LIMIT

Safety :

o Flammable gas storage area should be ventilated;

o Note the specific gravity of gas; lighter gas is

concentrated in the ceiling, heavy gas at the base;

o Ventilation (natural or mechanical) must be able to

limit the concentration of flammable gas up to 25%

LFL.

168

FLAME TEMPERATURE

Temperature of flue gas leaving the reaction zone, Tf;

Premixed flame :

o Mixture composition is easily known ;

o Mixture enters the flame with a fixed temperature and pressure;

o Flame temperature is easily calculated from the thermodynamic

properties of mixtures;

Diffusion flame :

o Composition of the mixture is difficult to be known so it is difficult to

calculate the flame temperature;

o Because the flame is produced at stoichiometric interphase, the

maximum flame temperature is high (near the adiabatic flame

temperature)

169

FLAME TEMPERATURE

Flame temperature of fuel/ air ~ 2000 K. Near the fllamability limit,

lower temperature 1400-1500 K.

Flame temperature of stable gas phase (homogeneous) combustion

: > 1400oC (between 1500 and 1900oC); This high temperature

heats fuel and air which comes in conduction, convection or

radiaton.

Maximum temperature of premixed flame can be controlled through

the air dilution.

The maximum temperature of difussion flame is higher (~2000oC for

natural gas and ~2200oC for diesel) because of the stoichiometry on

the flame front. As a result, diffusion flame is more stable.

170

HIGH FLAME TEMPERATURE

Advantages :

o Better process

o Stable flame

o High efficiency of energy conversion

Disadvantages :

o At high temperature, nitrogen molecule in the air reacts with oxygen to

form Nox; the higher temperature, the reaction becomes faster.

o At high temperature, production of soot increases

o The maximum flame temperature in combustion system should be

limited

171

EQUILIBRIUM NO CONCENTRATION

172

FLAME

173

Laminar premixed flame of methane: Little of fuel-rich ( = 1, left), fuel-rich and sooting ( > 1,middle), and

diffusion flame (right).

Flame luminosity increases with raising of equivalence ration because the production of soot increases.

FLAME

174

Picture shows the probe of sampling withdrawal from the flame.

Measurement of samples :

o Provide information of chemical flame;

o Understand the chemical and mechanism of combustion

Flame contains hundreds of intermediate substances.

CONCENTRATION PROFILE OF LAMINER PREMIXED FLAME

175

Concentration profile of species in the fuel rich methane

flame (premixed flame) along the flame or on the surface

of the burner.

CONCENTRATION PROFILE OF SPECIES THROUGH FRONT OF LAMINER DIFFUSION FLAME

176

Ethylene from left diffuses oxygen from right and

completed in the flame zone when the peak temperature

(1600oC) is reached.

BEHAVIOUR OF PREMIXED FLAME AND DIFFUSION FLAME

177

Intermediate behavior in diffusion flame and premixed

flame is same.

Contact pattern of fuel and oxidant in diffusion flame

and premixed flame is very different.

Reaction mechanism premix flame and diffusion flame

is same.



SUBJECT :

IGNITION PROCESS


178

IGNITION PROCESS

179

Rapid reaction between fuel-oxygen

Type :

o Otoignition (in diesel engine);

Thermal Ignition

Chain Ignition

o Ignition of induction (in gasoline engine)

OTOIGNITION-CHAIN IGNITION

180

Vacum empty vessel (T0 and P0);

Entering reactant directly reach P and T0;

Evolution of temperature (figure b);

After a period of time (ignition delay time), the temperature rise drastically, fuel is

burnt, then temperature drops again because of heat loss through the walls;

It is called otoignition or spontaneous ignition (chain).


181

Influenced by chain branching process ;

RH + O2 R* + HO2 (initiation stage, slow)

RH + HO2 R* + H2O2 (propagation)

H2O2 OH + OH (chain branching)

RH + OH R* + H2O (propagation, fast)


182

During the ignition delay period, the population of radical

pool increases exponentially. However, the amount of

consumed fuel and realesed heat is too small to be

detected.

Chain branching reaction occurs during the induction

time, while the temperature remains constant.

Finally, the radical pool becomes large enough to

consume most of the fuel, and ignition occurs fast

DEFINITION OF IGNITION DELAY TIME

183

Fuel consumption;

Formation of CO;

Formation of OH;

The increase of pressure at constant volume;

The increase of temperature in adiabatic vessel.

OTOIGNITION-THERMAL IGNITION

184

Waktu

log T

Occurs at high temperature (or

high pressure);

There is enough energy to

initiate the mixture ;

R1C CR2 R1C*+ *CR2

(initiation);

Temperature directly increases.

IGNITION OF INDUCTION

185

Caused by sources of ignition (electric leap, matches,

etc.);

Sources of ignition heats the local volume of mixture so it

has otoignition (thermal or chain);

Flame spreads and heats other mixture volume;

Combustion occurs in self-sustained.

OTOIGNITION OF METHANE

Green : lean; Red: stoichiometric; Blue : rich

186

OTOIGNITION OF N-PENTANE

187

OTOIGNITION OF N-HEPTANE

188

OTOIGNITION OF SOME N-PARAFIN

189

Safety aspects: otoignition occurs when a flammable substance (coal, oil)

are stored.

Controlling phenomena in diesel engines: the fuel is injected into the air

with high pressure and temperature; combustion starts spontaneously

after ignition delay time.

Controlling phenomena in gasoline engine: when the flame propagates

along the cylinder, or when the compression, the increased

pressure will heat the unburned mixture so it can ignite spontaneously (it is

called as knock).

Other applications of combustion (example : gas turbines): fuel

and air are mixed before reaching the combustion chamber so it is

dangerous if otoignition occurs; structural damage occurs.

Because of its role, the otoignition time of a mixture needs to be calculated.

o It requires chemical description, initial temperature and pressure and

other parameters of flow, such as heat loss.

ROLE OF OTOIGNITION

190

When a mixture is stagnantly homogeneous, adiabatic, constant

volume, there is no convection and diffusion; energy equation

become:

The last equation describes the evolution of temperature versus

time.

191

OTOIGNITION WITHOUT HEAT LOSS

Initial temperature (T at t = 0)

Pressure (density)

Concentration of fuel and oxygen.

192

SENSITIVITY OF IGNITION DELAY TIME

Limit of temperature-pressure that separates the fast

reaction region and slow reaction region for certain ratio

of fuel-oxygen,

It is applied for certain equivalent ratio.

193

IGNITION LIMIT

At a certain P and T, mixture of H2-O2 in the vessel will burst spontaneously after

ignition delay time.

If the pressure is lowered to P1 (P1 <P), the reaction occurs slowly,

no spontaneous explosion;

If the pressure is increased to P2, (P2> P), reaction occurs slowly, no

spontaneous explosion;

This phenomenon is illustrated in the diagram of pT explosion. 194

IGNITION LIMIT OF H2-O2

T = 800 K, p <5 mbar not

ignite

Reactive radicals, which are

formed in the gas

phase diffuses into the

wall to join back into a stable

species.

At low P, the rate of diffusion is

faster than the rate of radical

production in the gas

phase so that the ignition does

not occur.

195


T/K

p/bar

Reaksi lambat

eksplosi

eksplosi

Reaksi lambat

800 K

If at T = 800 K, the pressure is

raised above the first ignition limit,

the rate of radical diffusion to the

wall is reduced to less than the

rate of radical production

spontaneous ignition occurs.

The first explosion limit depends

on the surface of the vessel in

which reaction of chain

termination occurs.

196


T/K

p/bar

Reaksi lambat

eksplosi

eksplosi

Reaksi lambat

800 K

If at T = 800 K, the pressure is

increased to 100 mbar, the second

explosion limit occurs because

of competition between branching a

nd chain termination reactions in the

gas phase.

At pressures below 100 mbar, the

chain branching reaction

H + O2 OH + O

OH and O react rapidly with the

fuel that produces H then reacts

according to the above reaction

produces more radical. Radicals

increases with an exponential rate

(the base of the explosion).

197


T/K

p/bar

Reaksi lambat

eksplosi

eksplosi

Reaksi lambat

800 K

The second explosion limit occurs

because the chain branching reaction

competes with trimolecular reaction.

H + O2 + M HO2 + M

produces HO2 radical that has

moderate reactivity (chain

termination).

The increase of trimolecular reaction

rate with pressure is faster than

bimolecular reaction. At a certain

pressure range, the rate of timolecular

reaction is larger than its competitors

bimolecular reaction rate (slow

reaction).

198


T/K

p/bar

Reaksi lambat

eksplosi

eksplosi

Reaksi lambat

800 K

199


T/K

p/bar

Reaksi lambat

eksplosi

eksplosi

Reaksi lambat

800 K

At higher pressures, there was third

explosion limit (thermal explosion li

mit) that occurs because of

competition between the heat

generated by chemical reaction and

heat loss to the vessel wall.

If the pressure is increased further

then the heat production per

increased volume so explosion

occurs at high pressure.

200

IGNITION LIMIT OF HYDROCARBONS

It is unlike the third explosion limit;

Because of the additional chemical

processes (such as the formation

of peroxides);

o Ignition occurs after the pulse

emission of short light

(multistage ignition)

o Combustion occurs at low

temperatures (cool flame).

201

IGNITION LIMIT OF HYDROCARBONS

Inhibitors of ignition in the area of cold flame (eg. CH4/O2)

o CH3 + O2 CH3O2 (a)

o CH3O2 + CH4 CH3OOH + CH3 (b)

o CH3OOH CH3O + OH (c)

The reactions above form mechanism of chain branching

that causes ignition.

Rising of temperature could shift the equilibrium of

reaction (a). At higher temperatures CH3O2 is

decomposed; chain branching step (c) is no longer fed by

the reaction of (a).

It is called failure of chain when T rises or negative

temperature coefficient (NTC).

IGNITION LIMIT OF HYDROCARBON

0

2

4

6

8

10

12

200 300 400 500 600 700 800

Temperatur (C)

Tekanan

(atm)

Propana

Etana

Metana

202

Pressure

H2 COMBUSTION CHEMISTRY

203

SENSITIVITY ANALYSIS OF H2 COMBUSTION

204

OH CONCENTRATION PROGRESS

205

IGNITION DELAY TIME H2

206

FLAME VELOCITIES OF H2

207

PROFILE OF H2,O2,AND H2O

208

COMBUSTION CHEMISTRY OF CH4

209

Combustion chemistry of CH4 is very complex.

Molecules of fuel, before produce CO2 and H2O, undergo

Molekul bahan bakar, sebelum menghasilkan CO2 dan H2O,

undergo a complex series of reaction steps that makes a lot of

intermediate.


210

The reaction series of CO2 formation :

Oxidation of CO

o The last series in the combustion of CH4 and other hydrocarbons.

o Occurs through reaction with OH,

Radical of OH (and H, O)

o Free radicals are important in the process of burning flame,

o In the same time, it forms the radical pool in the flame through the chain

branching reactions.


211

Chain branching reaction:

o Basis of H2 combustion,

o Important submechanism in combustion of all hydrocarbon fuels.

Destruction of the fuel and the formation and destruction of all

intermediate occurs through free radical reactions.


212

Important reactions in the flame:

o Radicals reaction is caused by radical concentration at high flame

front.

o HC fuel is attacked by active radicals of H,O, and OH.

o Alkyl radicals decompose into smaller alkyl radical and the alkene.

o Smallest alkyl radical (CH3 and C2H5) is thermally decomposed

relatively slow and compete with the recombination reaction and the

oxidation reaction with O or O2 (the rate controlling step in the flame

of alkanes and alkenes).


213

Methane in the flame is attacked by the radicals of

H, OH, and O,


214

CH3 radicals have to undergo dehydrogenated

recombination to form various species of C2 which have

high enough concentration in the fuel-rich flame.


215

Vinyl radical in the fuel-lean mixture is oxidized become

CO and CO2,


216

In the fuel-rich mixture, growth of molecule become

aliphatic, aromatic, and polyaromatic C3-C6 occurs.


217

The reaction of benzene and naphthalene (C10H8) produces

greater polyaromatic hydrocarbons(PAH).

Subsequent growth of hydrocarbon byproducts forms species

with very low vapor pressure. This species condense and

undergo dehydrogenation to form a heterogeneous core

(liquid) for the formation of soot. Soot is an important

characteristic of the fuel-rich flame.

218

SCHEME OF SOOT FORMATION THROUGH

PAH

FLOW REACTION ANALYSIS OF CH4-AIR

STOICHIOMETRY

219

SENSITIVITY ANALYSIS OF CH4-AIR STOICHIOMETRICALLY

220

IGNITION DELAY TIME OF CH4-O2

Hijau: lean; merah: stoikiometri; biru: rich

221

Green : lean, Red: stoichiometric; Blue : rich

FLAME VELOCITIES OF CH4

222

DEPENDENCE OF FLAME VELOCITY ON MIXTURE COMPOSITION

223

DEPENDENCE OF FLAME VELOCITY ON PRESSURE AND TEMPERATURE

224

CHEMICAL OF FLAME ON LONG HYDROCARBON

225

Chemical combustion of long hydrocarbon (propane, butane,

gasoline) involves reactions that have been discussed before.

First, fuel decomposes into reactive intermediates and low-

molecular-mass fragments such as methane and ethylene. This

decomposition products determine the chemical flame and emission

from tools for combustion of long hydrocarbon fuel.

Chemical aspects of combustion is very helpful in developing

a detailed kinetic mechanisms for combustion of all kinds

hydrocarbon fuels.


226

Although the detailed mechanism of chemical kinetic for the

long hydrocarbon consists of several hundreds species which

are involved in thousands of elementary chemical reactions,

but the types of its reactions are limited.

Based on these observations, we can formulate all reactions

that occur in combustion and oxidation of long hydrocarbons

together with its rate coefficient by using simple rules.

Each rule describes a specific type of reaction.


227

Types of reactions:

o Decomposition of hydrocarbons;

o Abstraction of H atom by active radicals;

o Radical break in the β position,

o Abstraction of internal H atom (isomerization),

o Addition of radical to the O2 molecule,

o Termination of O-O bond;

o Addition of radicals to the double bond.


228

Rate coefficient (C> 4) depends on:

o Radically legible abstraction of H atom from alkanes,

alkenes, aldehydes, ketones or cyclicether;

o Type of abstracted H atom (primary, secondary,

tertiary);

o Amount of equivalent H atoms ;

o Size of the intermediate ring structure (5, 6, 7 or 8

members).


229


230

Reaction group :

o Reaction at high temperature;

o Reactions at low temperatures;

REACTION AT HIGH TEMPERATURE

231

Molecular decomposition of alkanes

o C-H> C-C,

o Relative strength of CH and CC: primary> secondary>

tertiary.


232

Abstraction of H atom from alkanes :

o The rate coefficient depends on the

legible abstraction radical,

type of abstracted H atom,

amount of equivalent H atoms.

o Tertiary C-H < seondary C-H <primary C-H;

o Abstraction of H atom from fuel through the attack of oxygen molecules

which act as initiation reaction. At low temperature, the reaction is rather

slow because of high activation energy (> 167 kJ / mol). However, the

reaction is still running as the R radical starts the chain.


233

Decomposition of alkyl radical

o Termination of bond at β position to the radical site.

o It is important only at high temperature (T > 900 K) due to high

activation energy.

o At low temperature, the important reaction is the addition of alkyl

radical to oxygen molecule (zero activation energy).


234

Isomerization of alkyl radical

o Alkyl radical transfers H atom from one position to radical

position to generate new radical location.

o Coefficient of rate depends on :

Energy barrier of chain strain, which is described in amount

of atoms in chain structure at transition state (including H),

Type of abstracted H atom,

Amount of equivalent H atom.


235

Oxidation of alkyl radical to form alkene

o Irreversible reaction,

o Energy barrier is not large,

o It forms alkene conjugate.

o Rate coefficient depends on

Type of abstracted H atom;



236

Decomposition of alkene

o Occurs through various path of reaction;

o The most important is reaction that forms allyl radical

(low activation energy ~ 290 kJ/mole),


237

Abstraction of allyl H atom

vinyl

allyl

alkenyl


238

Abstraction of allyl H atom

o It forms resonance-stabilized radicals which further break

in a position to produce, for example, 1-3-butadiene.

o Rate coefficient depends on :

Type of allyl H atom;



239

Abstraction of vinyl H atom

o Two types of vinyl H :

Secondary

Tertiary

o The Reaction is more difficult than allyl H atom.

o Rate coefficient depends on :

Type of vinyl H atom



240

Abstraction of alkenyl H atom

o It has role in the reaction of

long chain alkene that produce

products which is observed in

experiments such as dialkene

(eg. C5H8)

o Rate coefficient :

It is same as abstraction of H atom from alkane;

It depends on type of H atom;

It depends on amount of equivalent H atom.


241

Addition of H to the double bond

o Very exothermic;

o Inverse of alkyl radical decomposition at position.


242

Addition of CH3 to the double bond

o Inverse of alkyl radicals decomposition at position.


243

Addition of O to the double bond

o It forms ketyl radical dan short alkyl radical.


244

Addition of OH to the double bond

o It forms aldehyde or ketone.


245

Addition of HO2 to the double bond

o It forms hydroperoxy alkyl radical, R’OOH, which then

decomposes to form cyclic ether, aldehyde or ketone.


246

Reaction of retroena

o Reaction of 1,5 hydrogen shift which is followed by

dissociation;

o It forms two short chain alkene.

o For example, reaction of 1-heptane produces 1-

butene and propene.


247

Isomerization of alkenyl radical

o It produces resonance-stabilized radicals.

o The fastest isomerization : involving transition state

which contains five or six atoms.

o At 1100 K, isomerization of alkenyl radical produces

allyl radical 5-10 times faster than termination of β.


248

Decomposition of ally radical

o Bond termination at β position from radical position;

o It produces dialkene (eg 1,3-butadiene and 1,3-

pentadiene).


249

Decomposition of vinyl radical

o Bond termination at β position from radical position to

produce dialkene;

o It produces alkyne


250

Decomposition of alkenyl radical

o This is an important reaction if isomerization of alkenyl

radical may not occur because the alkenes is too short.

o It occurs through the termination of β that produces :

Dialkene and alkyl radical;

Alkene and short alkenyl radical.


251

REACTION AT LOW TEMPERATURE

252

Addition of alkyl on O2

o At T <, reacton of alkyl termination at β position and

isomerization is slow because Ea (113-167 kJ/mole).

o Most important alkyl reaction at T< is the addition of alkyl on O2.

Exothermic, reversible and Ea <<< .

If T , reaction to the left; RO2 looses and its concentration

is very low.


253

Isomerization of alkylperoxy

o Through the transfer of H (1,4-, 1,5-,1,6- dan 1,7-)

o Coefficient of reaction rate depends on

Energy barrier of chain strain (5-, 6-, 7- and 8- members)

Type of abstracted H (primary, secondary, tertiary),

Amount of equivalent H.


254

Abstraction of H from alkane by alkylperoxy

o Coefficient of reaction rate depends on :

Type of abstracted H,

Amount of equivalent H.


255

Reaction of alkylperoxy with HO2

Reaction of alkylperoxy with H2O2


256

Termination of O-O hydroperoxide bond

Decomposition of alkoxy


257

Addition of alkyl hydroperoxy on O2

Decomposition of alkyl hydroperoxy


258

Termination of O-O pada alkyl hydroperoxy

Oxidation of alkyl hydroperoxy


259

Formation of cyclic ether from alkyl hidroperoxide

o Consists of :

Termination of O-O,

Formation of cyclic ether.

o Ea depends on the size of the cyclic ring.

o Cyclic ether : oksirana, oksetana, tetrahidrofurana,

tetrahidropirana.


260

Isomerization of alkyl peroxide hidroperoxide.


261

Termination of O-O at alkyl dihidroperoxide

Decomposition of ketohydroperoxide


262

Decomposition of O=R″O●

Abstraction of H from cyclic ether


263

Abstraction of H from aldehyde or ketone

Decomposition of Ketyl

264

N-PENTANE 185 species,

1186 elementary reactions

265

N-HEPTANE 486 species,

2008 elementary reactions.

266



267



268



269



270

N-HEPTANE

486 species,


271

N-HEPTANE

486 species,


272

N-HEPTANE

486 species,


273

N-HEPTANE

486 species,


274

ISO-OCTANE

950 species,


275

ISO-OCTANE

950 species,


276

ISO-OCTANE

950 species,


277

ISO-OCTANE

950 species,


278

ISO-OCTANE

950 species,


279

ISO-OCTANE

950 species,


280

ISO-OCTANE

950 species,


281

N-DECANE 1253 species,


282



283



284



285

1253 species,


286

N-DECANE



287



288


N-heptane

289


N-heptane

290


N-heptane

291

N-heptane

292


N-heptane

293


N-PARAFIN

294

PATHWAYS OF CO REACTION

295


296


297

PATHWAYS OF CO2 REACTION

298


299


At 1250 K

300

PATHWAYS OF CH2O REACTION

301

PATHWAYS OF CH2O REACTION

302

NITROGEN FORMATION

303

In the flame, nitrogen and oxygen molecules interact each

other according to the mechanism of Zeldovich or thermal

NO,

The second reaction will be important if it is above 1500oC.

Diffusion flame is very vulnerable to produce high

concentration of NO at high flame temperature.

NITROGEN FORMATION

304

Formation of NO also occurs through the mechanism of NO,

and from source of nitrogen in the fuel.

In the mechanism of NO, radical reaction of CH with N2

produces NO.

NITROGEN FORMATION

305

Mechanism of this reacton also plays a role in the

process of thermal deNOx to remove NO from

combustion products using NH3.

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Teknik Pembakaran English Version