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



Termodinámica II I

Contenido

1. Introduction

2. A/F ratio

3. Energy Balance

4. Pollution issues

2


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Termodinámica II I

Contenido

1. Introduction

2. A/F ratio

3. Energy Balance

4. Pollution issues

3



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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Termodinámica II I

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[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).



Termodinámica II I

Combustion

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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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Termodinámica II I

Contenido

1. Introduction

2. A/F ratio

3. Energy Balance

4. Pollution issues

7



Termodinámica II I

Stoichiometric reaction

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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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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]



Termodinámica II I

Stoichiometric reaction

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What happens when air is humid?

Air with = 60% and dry bulb temperature of 20⁰C @ Quito atm. pressure.

SOLUTION


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Termodinámica III

11dry air



Termodinámica III

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Fuel: natural gas


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Termodinámica III

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If 10% of excess air is required, then:



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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Termodinámica III

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If the products are cooled down at 30

⁰C, what is the amount of water thatwil condense if 10% of excess air is required?



Termodinámica III

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Termodinámica II I

Contenido

1. Introduction

2. A/F ratio

3. Energy Balance

4. Pollution issues

17



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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Termodinámica II I

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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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Termodinámica III

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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



Termodinámica III

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First-Law Analysis

Steady Flow Systems

[1]


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Termodinámica II I

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First-Law Analysis Closed Systems

[1]



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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Termodinámica II I

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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



Termodinámica II I

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Termodinámica II I

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[1]

Adiabatic Flame Temperature is the máximum temperature whencombustión is complete and no heat is lost to the surroundings.



Termodinámica II I

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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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Termodinámica II I

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Enthalpies @ temperature trial and error.



Termodinámica II I

Contenido

1. Introduction

2. A/F ratio

3. Energy Balance

4. Pollution issues

32


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Termodinámica II I

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[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.



Termodinámica II I

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[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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Termodinámica II I

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[3]

CO has affinity with hemoglobin and removes O2 from the blood.

Effects of CO exposure on humans.



Termodinámica II I

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cap 4 combustion

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