Lecture 16a Chemcial Reactions and Combstion
Download as PPTX, PDF2 likes361 views
This lecture discusses the chemical reactions involved in combustion and how fuel composition changes during these reactions. It covers the fundamentals of combustion, the types of fuels, pollution from vehicles, energy release from fuels, and the importance of balancing chemical reactions. Key concepts such as air-fuel ratios, ignition temperatures, and safety limits for pollutants are also outlined.
Lecture 16a Chemcial Reactions and Combstion
- 1. THERMODYNAMICS II LECTURE 16a Chemical Reactions and Combustion 1
- 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 2 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 3
- 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 4
- 5. Energy released from some common fuels 5 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 6
- 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! 8
- 9. Air-Fuel Ratio • Air to fuel ratio • Reciprocal : Fuel to Air Ratio • Lean and Rich mixture 9 Stoichiometric AFR Gasoline 14.7 : 1 Diesel 14.5 : 1 Gasoline Lean 15.0 : 1 Stoichiometric 14.7 : 1 Rich 13 : 1 Remember
- 10. 10
- 11. • Chemical equation of reaction is: • Comparing the coefficients of C, H, O and N2: 11
- 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: 12
- 13. 13