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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
DEPARTEMEN TEKNIK KIMIA UNIVERSITAS INDONESIA
COMBUSTION ENGINEERING
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
CLASSIFICATION OF EMISSION
20
CLASSIFICATION OF EMISSION
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
DEPARTEMEN TEKNIK KIMIA UNIVERSITAS INDONESIA
COMBUSTION ENGINEERING
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
DEPARTEMEN TEKNIK KIMIA UNIVERSITAS INDONESIA
COMBUSTION ENGINEERING
SUBJECT :
COMBUSTION CONTROL
Oleh : Dr.-Ing Donni Adinata, ST., M. Eng. Sc
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
LAMINAR (TURBULENT) PREMIX LAMINAR
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
LAMINAR (TURBULENT) PREMIX LAMINAR
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
DEPARTEMEN TEKNIK KIMIA UNIVERSITAS INDONESIA
COMBUSTION ENGINEERING
SUBJECT :
COMBUSTION
STOICHIOMETRY
Oleh : Dr.-Ing Donni Adinata, ST., M. Eng. Sc
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
COMBUSTION STOICHIOMETRY
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
NON-STOICHIMETRY MIXTURE
Equivalence ratio, :
– < 1 poor fuel;
– > 1 rich fuel.
69
NON-STOICHIMETRY MIXTURE
Air-fuel relative ratio, 1 /:
– AFRactual/AFRstoichiometry;
– Also called an equivalence ratio of oxidizing agent
Percent theoretical air, 100 / ;
Percent excess air, EA:
70
COMBUSTION STOICHIOMETRY
a, a1, a2 …. = coefficient;
= equivalence ratio;
Other species can be added on the right side;
71
COMBUSTION STOICHIOMETRY
If = 1, so complete reaction
become :
72
COMBUSTION STOICHIOMETRY
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
COMBUSTION STOICHIOMETRY
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
DEPARTEMEN TEKNIK KIMIA UNIVERSITAS INDONESIA
COMBUSTION ENGINEERING
SUBJECT :
FIRST LAW OF
THERMODYNAMICS
Oleh : Dr.-Ing Donni Adinata, ST., M. Eng. Sc
79
ENERGY BALANCE AND ENTHALPY
80
ENERGY BALANCE AND ENTHALPY
Constant Volume, TR = TP = T’
becomes
Heat reaction at constant volume
in T’ (per mol)
81
ENERGY BALANCE AND ENTHALPY
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’ :
ENERGY BALANCE AND ENTHALPY
83
ENERGY BALANCE AND ENTHALPY
Combustion product is H2O in vapor and liquid phase
84
ENERGY BALANCE AND ENTHALPY
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
Adiabatic combustion in constant pressure :
95
ADIABATIC COMBUSTION
Adiabatic flame temperature
96
DEPARTEMEN TEKNIK KIMIA UNIVERSITAS INDONESIA
COMBUSTION ENGINEERING
SUBJECT :
CHEMICAL KINETICS
Oleh : Dr.-Ing Donni Adinata, ST., M. Eng. Sc
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
REACTION RATE COEFFICIENT DEPENDENCE ON PRESSURE
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 COEFFICIENT DEPENDENCE ON PRESSURE
reaction rate
** *a a u
d Ak A M k A M k A
dt
111
REACTION RATE COEFFICIENT DEPENDENCE ON PRESSURE
Assumption: the concentration of A * in is quasi-steady
state
** *a a u
d Ak A M k A M k A
dt
112
REACTION RATE COEFFICIENT DEPENDENCE ON PRESSURE
Extreme condition
o P <<<
[M] <<<
Border stage : the activation
o P >>>
[M] >>>
Border stage: the decomposition of A *.
113
Second order
First order
REACTION RATE COEFFICIENT DEPENDENCE ON PRESSURE
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
SENSITIVITY ANALYSIS
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
DEPARTEMEN TEKNIK KIMIA UNIVERSITAS INDONESIA
COMBUSTION ENGINEERING
SUBJECT :
FLAME TEMPERATURE
Oleh : Dr.-Ing Donni Adinata, ST., M. Eng. Sc
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
CHEMICAL EQUILIBRIUM
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
CHEMICAL EQUILIBRIUM
Effect of temperature on the equilibrium:
- Kirchhoff equation
- In a specific scale
- Substitution of the specific heat:
- The result:
dTchd pˆˆ0
RdTTaTaTaTaahd 4
5
3
4
2
321
0ˆ
148
CHEMICAL EQUILIBRIUM
Effect of temperature on the equilibrium:
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
CHEMICAL EQUILIBRIUM
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
CHEMICAL EQUILIBRIUM
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
DEPARTEMEN TEKNIK KIMIA UNIVERSITAS INDONESIA
COMBUSTION ENGINEERING
SUBJECT :
SIMPLE
THERMOCHEMICAL MODEL
Oleh : Dr.-Ing Donni Adinata, ST., M. Eng. Sc
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
SIMPLE THERMOCHEMICAL MODEL
Fuel mass balance:
Energy balance:
Equations of motion:
156
SIMPLE THERMOCHEMICAL MODEL
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
SIMPLE THERMOCHEMICAL MODEL
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
COMPLETE THERMOCHEMICAL MODEL
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
COMPLETE THERMOCHEMICAL MODEL
DEPARTEMEN TEKNIK KIMIA UNIVERSITAS INDONESIA
COMBUSTION ENGINEERING
SUBJECT :
FLAME
Oleh : Dr.-Ing Donni Adinata, ST., M. Eng. Sc
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.
DEPARTEMEN TEKNIK KIMIA UNIVERSITAS INDONESIA
COMBUSTION ENGINEERING
SUBJECT :
IGNITION PROCESS
Oleh : Dr.-Ing Donni Adinata, ST., M. Eng. Sc
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).
OTOIGNITION-CHAIN IGNITION
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)
OTOIGNITION-CHAIN IGNITION
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
IGNITION LIMIT OF H2-O2
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
IGNITION LIMIT OF H2-O2
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
IGNITION LIMIT OF H2-O2
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
IGNITION LIMIT OF H2-O2
T/K
p/bar
Reaksi lambat
eksplosi
eksplosi
Reaksi lambat
800 K
199
IGNITION LIMIT OF H2-O2
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.
COMBUSTION CHEMISTRY OF CH4
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.
COMBUSTION CHEMISTRY OF CH4
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.
COMBUSTION CHEMISTRY OF CH4
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).
COMBUSTION CHEMISTRY OF CH4
213
Methane in the flame is attacked by the radicals of
H, OH, and O,
COMBUSTION CHEMISTRY OF CH4
214
CH3 radicals have to undergo dehydrogenated
recombination to form various species of C2 which have
high enough concentration in the fuel-rich flame.
COMBUSTION CHEMISTRY OF CH4
215
Vinyl radical in the fuel-lean mixture is oxidized become
CO and CO2,
COMBUSTION CHEMISTRY OF CH4
216
In the fuel-rich mixture, growth of molecule become
aliphatic, aromatic, and polyaromatic C3-C6 occurs.
COMBUSTION CHEMISTRY OF CH4
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.
CHEMICAL OF FLAME ON LONG HYDROCARBON
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.
CHEMICAL OF FLAME ON LONG HYDROCARBON
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.
CHEMICAL OF FLAME ON LONG HYDROCARBON
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).
CHEMICAL OF FLAME ON LONG HYDROCARBON
229
CHEMICAL OF FLAME ON LONG HYDROCARBON
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.
REACTION AT HIGH TEMPERATURE
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.
REACTION AT HIGH TEMPERATURE
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).
REACTION AT HIGH TEMPERATURE
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.
REACTION AT HIGH TEMPERATURE
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;
Amount of equivalent H atom.
REACTION AT HIGH TEMPERATURE
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),
REACTION AT HIGH TEMPERATURE
237
Abstraction of allyl H atom
vinyl
allyl
alkenyl
REACTION AT HIGH TEMPERATURE
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;
Amount of equivalent H atom.
REACTION AT HIGH TEMPERATURE
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
Amount of equivalent H atom.
REACTION AT HIGH TEMPERATURE
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.
REACTION AT HIGH TEMPERATURE
241
Addition of H to the double bond
o Very exothermic;
o Inverse of alkyl radical decomposition at position.
REACTION AT HIGH TEMPERATURE
242
Addition of CH3 to the double bond
o Inverse of alkyl radicals decomposition at position.
REACTION AT HIGH TEMPERATURE
243
Addition of O to the double bond
o It forms ketyl radical dan short alkyl radical.
REACTION AT HIGH TEMPERATURE
244
Addition of OH to the double bond
o It forms aldehyde or ketone.
REACTION AT HIGH TEMPERATURE
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.
REACTION AT HIGH TEMPERATURE
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.
REACTION AT HIGH TEMPERATURE
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.
REACTION AT HIGH TEMPERATURE
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).
REACTION AT HIGH TEMPERATURE
249
Decomposition of vinyl radical
o Bond termination at β position from radical position to
produce dialkene;
o It produces alkyne
REACTION AT HIGH TEMPERATURE
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.
REACTION AT HIGH TEMPERATURE
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.
REACTION AT LOW TEMPERATURE
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.
REACTION AT LOW TEMPERATURE
254
Abstraction of H from alkane by alkylperoxy
o Coefficient of reaction rate depends on :
Type of abstracted H,
Amount of equivalent H.
REACTION AT LOW TEMPERATURE
255
Reaction of alkylperoxy with HO2
Reaction of alkylperoxy with H2O2
REACTION AT LOW TEMPERATURE
256
Termination of O-O hydroperoxide bond
Decomposition of alkoxy
REACTION AT LOW TEMPERATURE
257
Addition of alkyl hydroperoxy on O2
Decomposition of alkyl hydroperoxy
REACTION AT LOW TEMPERATURE
258
Termination of O-O pada alkyl hydroperoxy
Oxidation of alkyl hydroperoxy
REACTION AT LOW TEMPERATURE
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.
REACTION AT LOW TEMPERATURE
260
Isomerization of alkyl peroxide hidroperoxide.
REACTION AT LOW TEMPERATURE
261
Termination of O-O at alkyl dihidroperoxide
Decomposition of ketohydroperoxide
REACTION AT LOW TEMPERATURE
262
Decomposition of O=R″O●
Abstraction of H from cyclic ether
REACTION AT LOW TEMPERATURE
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
N-HEPTANE 486 species,
2008 elementary reactions.
267
N-HEPTANE 486 species,
2008 elementary reactions.
268
N-HEPTANE 486 species,
2008 elementary reactions.
269
N-HEPTANE 486 species,
2008 elementary reactions.
270
N-HEPTANE
486 species,
2008 elementary reactions.
271
N-HEPTANE
486 species,
2008 elementary reactions.
272
N-HEPTANE
486 species,
2008 elementary reactions.
273
N-HEPTANE
486 species,
2008 elementary reactions
274
ISO-OCTANE
950 species,
3361 elementary reactions.
275
ISO-OCTANE
950 species,
3361 elementary reactions.
276
ISO-OCTANE
950 species,
3361 elementary reactions.
277
ISO-OCTANE
950 species,
3361 elementary reactions.
278
ISO-OCTANE
950 species,
3361 elementary reactions.
279
ISO-OCTANE
950 species,
3361 elementary reactions.
280
ISO-OCTANE
950 species,
3361 elementary reactions.
281
N-DECANE 1253 species,
4177 elementary reactions
282
N-DECANE 1253 species,
4177 elementary reactions
283
N-DECANE 1253 species,
4177 elementary reactions
284
N-DECANE 1253 species,
4177 elementary reactions
285
1253 species,
4177 elementary reactions
286
N-DECANE
N-DECANE 1253 species,
4177 elementary reactions
287
N-DECANE 1253 species,
4177 elementary reactions
288
SENSITIVITY ANALYSIS
N-heptane
289
SENSITIVITY ANALYSIS
N-heptane
290
SENSITIVITY ANALYSIS
N-heptane
291
N-heptane
292
SENSITIVITY ANALYSIS
N-heptane
293
SENSITIVITY ANALYSIS
N-PARAFIN
294
PATHWAYS OF CO REACTION
295
PATHWAYS OF CO REACTION
296
PATHWAYS OF CO REACTION
297
PATHWAYS OF CO2 REACTION
298
PATHWAYS OF CO2 REACTION
299
PATHWAYS OF CO2 REACTION
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.