1.2 Thermochemistry and Combustion Stoichiometry

Thermochemistry and combustion stoichiometry are crucial for understanding how fuels burn. These concepts help us figure out the energy released during combustion and the right mix of fuel and air for efficient burning.

We'll look at chemical equations, air-fuel ratios, and heating values. These ideas are key to designing better engines, furnaces, and power plants that use less fuel and produce fewer pollutants.

Chemical Reactions and Stoichiometry

Fundamentals of Chemical Equations and Ratios

• Chemical equations represent chemical reactions using symbols and formulas
• Balanced equations show equal numbers of atoms on both sides
• Stoichiometric ratio determines the relative quantities of reactants and products in a balanced equation
• Coefficients in balanced equations indicate the molar ratios of substances
• Equivalence ratio compares the actual fuel-to-oxidizer ratio to the stoichiometric ratio
  • Values less than 1 indicate a fuel-lean mixture
  • Values greater than 1 indicate a fuel-rich mixture
  • Value of 1 represents a stoichiometric mixture

Air-Fuel Relationships in Combustion

• Excess air refers to the amount of air supplied beyond the stoichiometric requirement
• Expressed as a percentage above the theoretical air needed for complete combustion
• Ensures complete fuel oxidation and reduces pollutant formation
• Air-fuel ratio (AFR) measures the mass of air to the mass of fuel in a combustion mixture
• AFR affects combustion efficiency, flame temperature, and emissions
• Typical AFR values range from 12:1 to 15:1 for gasoline engines
• Diesel engines operate with higher AFR values, typically between 18:1 and 70:1

Thermochemistry and Heating Values

Fundamental Concepts in Thermochemistry

• Heat of combustion measures the energy released when a substance undergoes complete combustion
• Expressed in units of energy per mole or mass of substance (kJ/mol or kJ/kg)
• Heat of formation represents the energy change when forming a compound from its constituent elements
• Standard heat of formation values are measured at 25°C and 1 atm pressure
• Calorific value indicates the amount of heat released during complete combustion of a fuel
• Measured using a bomb calorimeter under controlled conditions

Heating Value Classifications and Applications

• Lower Heating Value (LHV) assumes water vapor remains in gaseous form after combustion
• Excludes the latent heat of vaporization of water
• More relevant for applications where water vapor is not condensed (gas turbines, internal combustion engines)
• Higher Heating Value (HHV) includes the latent heat of vaporization of water
• Assumes all water vapor condenses to liquid form after combustion
• Used in applications where water vapor condensation is possible (boilers, furnaces)
• The difference between HHV and LHV depends on the hydrogen content of the fuel
• Fuels with higher hydrogen content show a larger difference between HHV and LHV (natural gas, propane)