Cap 4 Combustion
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Transcript of Cap 4 Combustion
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8/18/2019 Cap 4 Combustion
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Escuela Politécnica Nacional 19/06/20
ESCUELA POLITÉCNICA NACIONAL
FACULTAD DE INGENIERÍA MECÁNICA
Termodinámica II I
Combustion
Ing. José Luis Palacios E., M.Sc.
Quito, 04 de Junio 2015
1
ESCUELA POLITÉCNICA NACIONAL
FACULTAD DE INGENIERÍA MECÁNICA
Termodinámica II I
Contenido
1. Introduction
2. A/F ratio
3. Energy Balance
4. Pollution issues
2
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Escuela Politécnica Nacional 19/06/20
ESCUELA POLITÉCNICA NACIONAL
FACULTAD DE INGENIERÍA MECÁNICA
Termodinámica II I
Contenido
1. Introduction
2. A/F ratio
3. Energy Balance
4. Pollution issues
3
ESCUELA POLITÉCNICA NACIONAL
FACULTAD DE INGENIERÍA MECÁNICA
Termodinámica II I
Combustion
4
A chemical reaction during which a fuel is oxidized and a large quantity
of energy is released is called combustion.
When this chemical reaction occurs, the bonds within molecules of the
reactants are broken, and atoms and electrons rearrange to form
products.
[1]
[2]
[2]
mass of reactants = mass of the products
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Escuela Politécnica Nacional 19/06/20
ESCUELA POLITÉCNICA NACIONAL
FACULTAD DE INGENIERÍA MECÁNICA
Termodinámica II I
5
[1]
For simplicity:
gasoline → octane 8H18
diesel → dodecane 12H26
methanol → methyl alcohol H3OH
natural gas → methane H4
The ultimate analysis gives the composition on a mass basis in terms of
the relatives amounts of chemical elements (C, H₂ and S).
ESCUELA POLITÉCNICA NACIONAL
FACULTAD DE INGENIERÍA MECÁNICA
Termodinámica II I
Combustion
6
Rapid combustion of combustible elements of the fuel results in energy
release as combustion products are formed.
The 3 major combustible chemical elements in most common fuels are:
C, H₂ and S.
S is usually a relatively unimportant contributor to the energy released,
but it can be a significant cause of pollution and corrosion problems.[2]
[2]
[1]
Each mol of O2 reacts with 3.76 mol N2
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Escuela Politécnica Nacional 19/06/20
ESCUELA POLITÉCNICA NACIONAL
FACULTAD DE INGENIERÍA MECÁNICA
Termodinámica II I
Contenido
1. Introduction
2. A/F ratio
3. Energy Balance
4. Pollution issues
7
ESCUELA POLITÉCNICA NACIONAL
FACULTAD DE INGENIERÍA MECÁNICA
Termodinámica II I
Stoichiometric reaction
8
The mínimum amount of air needed for the complete combustión of a
fuel is called stocichiometric or theoretical air .
When a fuel is completely burned with theoretical air, no uncombined
oxygen is present in the combustión products gases.
The theoretical is also referred to as the chemically correct amount of
air, or 100% theoretical air. [1]
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ESCUELA POLITÉCNICA NACIONAL
FACULTAD DE INGENIERÍA MECÁNICA
Termodinámica II I
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Combustion is complete when all the carbon present in the fuel isburned to carbon dioxide, and all other combustible elements are fully
oxidized.
When these conditions are not fulfilled, combustion is incomplete.
A key parameter is air-fuel ratio (AF), which is the amount of air in the
reaction to the amount of fuel. [2]
[2]
[2]
ESCUELA POLITÉCNICA NACIONAL
FACULTAD DE INGENIERÍA MECÁNICA
Termodinámica II I
Stoichiometric reaction
10
What happens when air is humid?
Air with = 60% and dry bulb temperature of 20⁰C @ Quito atm. pressure.
SOLUTION
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ESCUELA POLITÉCNICA NACIONAL
FACULTAD DE INGENIERÍA MECÁNICA
Termodinámica III
11dry air
ESCUELA POLITÉCNICA NACIONAL
FACULTAD DE INGENIERÍA MECÁNICA
Termodinámica III
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Fuel: natural gas
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ESCUELA POLITÉCNICA NACIONAL
FACULTAD DE INGENIERÍA MECÁNICA
Termodinámica III
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If 10% of excess air is required, then:
ESCUELA POLITÉCNICA NACIONAL
FACULTAD DE INGENIERÍA MECÁNICA
Termodinámica III
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The dew-point temperature of the products is the temperature at which
the water vapor in the products starts to condense as the products are
cooled at constant pressure.
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ESCUELA POLITÉCNICA NACIONAL
FACULTAD DE INGENIERÍA MECÁNICA
Termodinámica III
15
If the products are cooled down at 30
⁰C, what is the amount of water thatwil condense if 10% of excess air is required?
ESCUELA POLITÉCNICA NACIONAL
FACULTAD DE INGENIERÍA MECÁNICA
Termodinámica III
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ESCUELA POLITÉCNICA NACIONAL
FACULTAD DE INGENIERÍA MECÁNICA
Termodinámica II I
Contenido
1. Introduction
2. A/F ratio
3. Energy Balance
4. Pollution issues
17
ESCUELA POLITÉCNICA NACIONAL
FACULTAD DE INGENIERÍA MECÁNICA
Termodinámica II I
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The enthalpy of formation is the energy released or absorbed when
the compund is formed from its elements.
The compound and elements all being at the standard reference state
(25°C and 1 atm).
[2]
[1]
[1]
‐ Heat is released
+ Heat must be added
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Escuela Politécnica Nacional 19/06/20
ESCUELA POLITÉCNICA NACIONAL
FACULTAD DE INGENIERÍA MECÁNICA
Termodinámica II I
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ESCUELA POLITÉCNICA NACIONAL
FACULTAD DE INGENIERÍA MECÁNICA
Termodinámica II I
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Heating value (HV) is the amount of heat released when a fuel is
burned completely in a esteady-flow process and the products are
returned to the state of the reactants.
The HV dependes on the phase of the H2O in the products.
Higher heating value (HHV) when the water in the products is in the
liquid form.
Lower heating value (LHV) when the water in the products is in the
vapor form.
[2]
[1]
[1]
[1]
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ESCUELA POLITÉCNICA NACIONAL
FACULTAD DE INGENIERÍA MECÁNICA
Termodinámica III
21
Determine the enthalpy of combustion (formation) of liquid octane(C8H18) at 25⁰C and 1 atm. Assume the water in the products is in the liquid
form.
stable elements
(liquid)
HHV
ESCUELA POLITÉCNICA NACIONAL
FACULTAD DE INGENIERÍA MECÁNICA
Termodinámica III
22
First-Law Analysis
Steady Flow Systems
[1]
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ESCUELA POLITÉCNICA NACIONAL
FACULTAD DE INGENIERÍA MECÁNICA
Termodinámica II I
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First-Law Analysis Closed Systems
[1]
ESCUELA POLITÉCNICA NACIONAL
FACULTAD DE INGENIERÍA MECÁNICA
Termodinámica II I
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Liquid propane (C3H8) enters a combustión chamber at 25°C at a rate
of 0.05 kg/min where it is mixed and burned with 50 percent excess
air that enters the combustión chamber at 7°C. An analysis of the
combustión gases reveals that all the hydrogen in the fuel burns to
H2O but only 90 percent of the carbón burns to CO2 with the remaining
10 percent forming CO. If the exit temperature of the combustión
gases is 1500K, determine the mass flow rate of air and the rate of
heat transfer from the combustión chamber.
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Escuela Politécnica Nacional 19/06/20
ESCUELA POLITÉCNICA NACIONAL
FACULTAD DE INGENIERÍA MECÁNICA
Termodinámica II I
25
3.76 → 3 4 3.76
2 6 4 → 5
5 ∗ 1.5 3.76 → 2.7 0.3 4 2.65 28.2
7.5 4.76 28.96
3 12 4 2
25.53
25.53
0.05
→ 1.18
ESCUELA POLITÉCNICA NACIONAL
FACULTAD DE INGENIERÍA MECÁNICA
Termodinámica II I
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Escuela Politécnica Nacional 19/06/20
ESCUELA POLITÉCNICA NACIONAL
FACULTAD DE INGENIERÍA MECÁNICA
Termodinámica II I
29
[1]
Adiabatic Flame Temperature is the máximum temperature whencombustión is complete and no heat is lost to the surroundings.
ESCUELA POLITÉCNICA NACIONAL
FACULTAD DE INGENIERÍA MECÁNICA
Termodinámica II I
30
Liquid octane enters the combustión chamber of a gas turbine
steadily at 1 atm and 25°C, and it is burned with air that enters the
combustion chamber at the same state. Determine the asiabatic flame
temperatura for complete combustion.
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Escuela Politécnica Nacional 19/06/20
ESCUELA POLITÉCNICA NACIONAL
FACULTAD DE INGENIERÍA MECÁNICA
Termodinámica II I
31
Enthalpies @ temperature trial and error.
ESCUELA POLITÉCNICA NACIONAL
FACULTAD DE INGENIERÍA MECÁNICA
Termodinámica II I
Contenido
1. Introduction
2. A/F ratio
3. Energy Balance
4. Pollution issues
32
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ESCUELA POLITÉCNICA NACIONAL
FACULTAD DE INGENIERÍA MECÁNICA
Termodinámica II I
33
[1]
Control of pollutant emissions is a major factor in the design of moderncombustion systems.
Pollutants of concern include particulate matter, such as soot, fly ash,
metal fumes, various aerosols.
EFFECTS OF POLLUTANTS
Primary air pollutants emitted directly from the source.
Secondary pollutants formed via reaction involving primary pollutants
in the atmosphere.
ESCUELA POLITÉCNICA NACIONAL
FACULTAD DE INGENIERÍA MECÁNICA
Termodinámica II I
34
[3]
Seinfeld indicates four principal effects of air pollutants in the
troposphere:
1. Altered properties of the atmosphere and precipitation.
2. Harm to vegetation.
3. Soiling and deterioration of materials.
4. Potential increase of morbidity (sickness) and mortality in humans.
Emission of Nox into the straosphere by
speed civil transpord aircraft is also an issue.
Concern here is about the catalytic destruction
of stratospheric ozone by NO.
ozone destruction
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ESCUELA POLITÉCNICA NACIONAL
FACULTAD DE INGENIERÍA MECÁNICA
Termodinámica II I
35
[3]
CO has affinity with hemoglobin and removes O2 from the blood.
Effects of CO exposure on humans.
ESCUELA POLITÉCNICA NACIONAL
FACULTAD DE INGENIERÍA MECÁNICA
Termodinámica II I
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