MET 401 Chapter 3 fuels_and_combustion


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MET 401 Chapter 3 fuels_and_combustion

  1. 1. MET 401 Power Plant Engineering Chapter 3Fuels and CombustionDr. Taib Iskandar Mohamad
  2. 2. Content1. Types of fuels2. Air-fuel ratio3. Heating values4. Heat of combustion / Enthalpy of Combustion5. Adiabatic Flame TemperatureTable A-17 – A-27 2
  3. 3. FUELS AND COMBUSTION Fuel: Any material that can be burned to release thermal energy. Most familiar fuels consist primarily of hydrogen and carbon. They are called hydrocarbon fuels and are denoted by the general formula CnHm. Hydrocarbon fuels exist in all phases, some examples being coal, gasoline (usually treated as octane C8H18), and natural gas.Most liquid hydrocarbon fuelsare obtained from crude oil bydistillation. 3
  4. 4. The oxidizer most often used in combustion processes is air. Why?On a mole or a volume basis, dry air is composed of 20.9% O2, 78.1% N2,0.9% Ar, and small amounts of CO2, He, Ne, H2.In the analysis of combustion processes, dry air is approximated as 21% O2and 79% N2 by mole numbers.Combustion is a chemical reaction during Each kmol of O2 in airwhich a fuel is oxidized and a large is accompanied by 3.76quantity of energy is released. kmol of N2. 4
  5. 5. The fuel must be brought above its ignitiontemperature to start the combustion. Theminimum ignition temperatures inatmospheric air are approximately 260°C forgasoline, 400°C for carbon, 580°C forhydrogen, 610°C for carbon monoxide, and630°C for methane.Proportions of the fuel and air must be in theproper range for combustion to begin. Forexample, natural gas does not burn in air inconcentrations less than 5% or greater thanabout 15%. The mass (and number of atoms) of each element is conserved during a chemical reaction. The total number of moles is not conserved during a chemical reaction.In a steady-flow combustion process, the components thatenter the reaction chamber are called reactants and thecomponents that exit are called products. 5
  6. 6. Air-fuel ratio (AF) is usuallyexpressed on a mass basis and is defined as the ratio of the m mass mass of air to the mass of fuel N number of moles for a combustion process M molar massFuel–air ratio (FA): The reciprocal of air–fuel ratio.The air–fuel ratio (AF) represents the amount of air usedper unit mass of fuel during a combustion process. 6
  7. 7. THEORETICAL AND ACTUAL COMBUSTIONPROCESSESComplete combustion: If all the carbon in the fuel burns to CO2, all thehydrogen burns to H2O, and all the sulfur (if any) burns to SO2.Incomplete combustion: If the combustion products contain any unburnedfuel or components such as C, H2, CO, or OH.Reasons for incomplete combustion: 1 Insufficient oxygen, 2 insufficientmixing in the combustion chamber during the limited time that the fuel and theoxygen are in contact, and 3 dissociation (at high temperatures). Oxygen has a much greater tendency to combine with hydrogen than it does with carbon. Therefore, the hydrogen in the fuel normally burns to completion,A combustion process is complete if all the forming H2O.combustible components of the fuel are burned tocompletion. 7
  8. 8. Stoichiometric or theoretical air: The minimum amount of air needed for thecomplete combustion of a fuel. Also referred to as the chemically correctamount of air, or 100% theoretical air.Stoichiometric or theoretical combustion: The ideal combustion processduring which a fuel is burned completely with theoretical air.Excess air: The amount of air in excess of the stoichiometric amount. Usuallyexpressed in terms of the stoichiometric air as percent excess air or percenttheoretical air.Deficiency of air: Amounts of air less than the stoichiometric amount. Oftenexpressed as percent deficiency of air.Equivalence ratio: The ratio of the actual fuel–air ratio to the stoichiometricfuel–air ratio.50% excess air = 150% theoretical air200% excess air = 300% theoretical air.90% theoretical air = 10% deficiency of air The complete combustion process with no free oxygen in the products is called theoretical combustion. 8
  9. 9. Predicting the composition ofthe products is relativelyeasy when the combustionprocess is assumed to becomplete.With actual combustionprocesses, it is impossible topredict the composition ofthe products on the basis ofthe mass balance alone.Then the only alternative wehave is to measure theamount of each componentin the products directly.A commonly used device to Determining the mole fraction of the CO2 inanalyze the composition of combustion gases by using the Orsat gascombustion gases is the analyzer.Orsat gas analyzer.The results are reported ona dry basis. 9
  10. 10. ENTHALPY OF FORMATION AND ENTHALPYOF COMBUSTIONDisregarding any changes in kinetic and potential energies, the energy changeof a system during a chemical reaction is due to a change in state and a changein chemical composition: When the existing chemical bonds are destroyed and new ones are formed during a combustion process,The microscopic form of energy of a usually a large amount of sensiblesubstance consists of sensible, latent, energy is absorbed or released.chemical, and nuclear energies. 10
  11. 11. Enthalpy of reaction hR : The difference between the enthalpy of the products ata specified state and the enthalpy of the reactants at the same state for acomplete reaction.Enthalpy of combustion hC : It is the enthalpy of reaction for combustionprocesses. It represents the amount of heat released during a steady-flowcombustion process when 1 kmol (or 1 kg) of fuel is burned completely at aspecified temperature and pressure.The enthalpy of formation hf : The amount of energy absorbed or released asthe component is formed from its stable elements during a steady-flow process ata specified state.To establish a startingpoint, we assign theenthalpy of formationof all stable elements(such as O2, N2, H2,and C) a value of zeroat the standardreference state of25°C and 1 atm. 11
  12. 12. Heating value: The amount of heatreleased when a fuel is burnedcompletely in a steady-flow processand the products are returned tothe state of the reactants. Theheating value of a fuel is equal tothe absolute value of the enthalpyof combustion of the fuel.Higher heating value (HHV):When the H2O in the products is inthe liquid form.Lower heating value (LHV): Whenthe H2O in the products is in thevapor form. The higher heating value of a fuel is equal to the sum of the lower heating value of the fuel and the latent heat of vaporization of the H2O in the products. For the fuels with variable composition (i.e., coal, natural gas, fuel oil), the heating value may be determined by burning them directly in a bomb calorimeter. 12
  13. 13. FIRST-LAW ANALYSIS OFREACTING SYSTEMSThe energy balance (the first-law) relations developedin Chaps. 4 and 5 are applicable to both reacting andnonreacting systems. We rewrite the energy balancerelations including the changes in chemical energies.Steady-Flow SystemsWhen the changes in kinetic and potential energies arenegligible, the steady-flow energy balance for achemically reacting steady-flow system: The enthalpy of a chemical component at a specified state 13
  14. 14. Taking heat transfer to the system and work done bythe system to be positive quantities, the energy balance relation isIf the enthalpy of combustion for a particular reaction is available:Most steady-flow combustion processes do not involve any work interactions.Also, combustion chamber normally involves heat output but no heat input: 14
  15. 15. Closed SystemsTaking heat transfer to the system and work doneby the system to be positive quantities, the generalclosed-system energy balance relation can beexpressed for a stationary chemically reacting An expression for theclosed system as internal energy of a chemical component in terms of the enthalpy.Utilizing the definition of enthalpy:The Pv terms are negligible for solids and liquids, and can bereplaced by RuT for gases that behave as an ideal gas. 15
  16. 16. ADIABATIC FLAME TEMPERATUREIn the limiting case of no heat loss to the surroundings (Q = 0), the temperatureof the products reaches a maximum, which is called the adiabatic flame oradiabatic combustion temperature. sinceThe determination of the adiabatic flame temperature by hand requires theuse of an iterative technique. The temperature of a combustion chamber becomes maximum when combustion is complete and no heat is lost to the surroundings (Q = 0). 16
  17. 17. The adiabatic flame temperature of a fuel depends on (1) the state of the reactants (2) the degree of completion of the reaction (3) the amount of air usedFor a specified fuel at a specified state burned with air at a specified state, the adiabatic flame temperature attains its maximum value when complete combustion occurs with the theoretical amount of air.The maximum temperature encountered in acombustion chamber is lower than the theoreticaladiabatic flame temperature. 17
  18. 18. Summary• Fuels and combustion• Theoretical and actual combustion processes• Enthalpy of formation and enthalpy of combustion• First-law analysis of reacting systems – Steady-flow systems – Closed systems• Adiabatic flame temperature 18
  19. 19. Example 1A fuel mixture of 20 percent by mass methane (CH4) and 80percent by mass ethanol (C2H6O) is burned completely withtheoretical air. The total flow rate of the fuel is 31 kg/s.a) Assuming complete combustion, determine the air/fuel ratio of the mixtureb) Then, required the required flow rate of airc) Explain how you calculate the enthalpy of combustion and the adiabatic flame temperature of this reaction. No calculation is required. 19
  20. 20. Example 2Calculate the enthalpy of combustion whenmethane is burned with air both at 25oC and 1atm, and the products were at 25oCRepeat this problem if 120% theoretical air isused. 20
  21. 21. Example 3• Determine the adiabatic flame temperature of the products when Hydrogen is burned with 20 percent excess air during a steady-flow combustion process. 21
  22. 22. Example 4 (assignment 1)• Determine the adiabatic flame temperature of the products when methane gas at 25oC and 1 atm reacts with 50% excess air at the same conditions in an adiabatic combustion chamber 22
  23. 23. Example 5• Determine the adiabatic flame temperature of the products when methane gas at 25oC and 1 atm reacts with 50% excess air at the same conditions in an adiabatic combustion chamber 23