General discussion about combustion and fuels. Thermodynamics II course taught at air university Islamabad by Sijal Ahmed.

1. THERMODYNAMICS II
LECTURE 16a
Chemical Reactions and Combustion
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2. Introduction
• In previous chapters we studied non-reacting mixtures, with
unchanged chemical composition.
𝑶 𝟐 + 𝑪𝑯 𝟒 ⟶ 𝑶 𝟐 + 𝑪𝑯 𝟒
• In this chapter we will consider mixture where mixture composition
will change due to chemical processes. i.e.
𝑶 𝟐 + 𝑪𝑯 𝟒 ⟶ 𝑪𝑶 𝟐 + 𝑯 𝟐 𝑶
• Chemical composition, chemical energy, combustion
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Methane combustion
Balanced Equation
Heat Out

3. Fuels and Combustion
• Any material that can be burned to release thermal energy is called a
Fuel.
• Hydrocarbon fuels 𝐶 𝑛 𝐻 𝑛
• Gasoline (petrol) : 𝐶8 𝐻18
• Diesel fuel : 𝐶12 𝐻26
• Methane : 𝐶𝐻4
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4. Fuels and Combustion
• Vehicles are major source of pollution : nitric oxides, carbon monoxide
and carbon dioxide (greenhouse gases)
• Safe limits:
– CO2 : 350 ppm in outdoor, up to 1000 ppm in indoor with good air exchange
– CO : 9 ppm max exposure
35 ppm, max exposure for 8 hours a day
800 ppm, death within 2 to 3 hours
12800 ppm, death within 1 to 3 minutes
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5. Energy released from some common fuels
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Cost of one liter of methanol = 1.2 $
Cost of one liter of gasoline = 1.8 $
Which implies that for 1 dollar you will get 15175 kJ and 17694 kJ of
energy from methanol and gasoline respectively.

6. Combustion
• A chemical reaction during which a fuel is oxidized and a large quality
of energy is released is called combustion.
• Air is common oxidizer, since it is free and readily
available. Nitron is inert gas and does not react.
• Let assume that we have 0.79 kmoles of N2 and 0.21 kmoles of O2
Divide both number of moles by 0.21 to normalize the value, we
get 1 kmol of o2 and 0.79/0.21 = 3.76 kmol of N2
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7. Combustion
• Components before reaction = Reactants
• Components exit after reaction = products
also called combustion products.
• 𝐶 + 𝑂2 ⟶ 𝐶𝑂2 C and O2 = Reactants and CO2 = Products
• Ignition temperature
– 260OC for gasoline
– 360OC for methane
– 210OC for Diesel 7

8. Balancing Reaction
• Mass is conserved during reaction
mass of reactants = mass of products
• No. of moles are not conserved!
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9. Air-Fuel Ratio
• Air to fuel ratio
• Reciprocal : Fuel to Air Ratio
• Lean and Rich mixture
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Stoichiometric AFR
Gasoline 14.7 : 1
Diesel 14.5 : 1
Gasoline
Lean 15.0 : 1
Stoichiometric 14.7 : 1
Rich 13 : 1
Remember

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11. • Chemical equation of reaction is:
• Comparing the coefficients of C, H, O and N2:
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12. • So the balanced equation is:
– Please note that, the number of moles of air on reactants side is equal to
20 + 20 × 3.76 = 95.2 𝑀𝑜𝑙𝑒𝑠
• Now air to fuel ratio is obtained as follows:
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