Combustion Chemistry Fundamentals

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Questions and Answers

What is the significance of the Gibbs function of formation in thermodynamics?

  • It represents the change in Gibbs function for the formation of a compound from its elements. (correct)
  • It measures the enthalpy change during a reaction.
  • It indicates the specific heat capacity of a substance.
  • It provides a direct measure of entropy in a system.

How is the Gibbs function calculated at a state other than the standard state?

  • By adding the specific Gibbs function change to the Gibbs function of formation. (correct)
  • By integrating the entropy values of all components.
  • By equating it with the overall energy balance of the system.
  • By using the initial Gibbs function and applying a uniform pressure change.

In the context of chemical exergy, what does it represent?

  • The energy released during spontaneous reactions.
  • The measure of how far the composition of a system is from the reference environment. (correct)
  • The total energy of all chemical components in a reaction.
  • The maximum work obtainable from a system at equilibrium.

What role does temperature play in determining entropy values in EES code?

<p>Temperature, along with partial pressure, is crucial for accurate entropy calculations. (B)</p> Signup and view all the answers

What is the relationship between mole fraction and unit issues in EES code?

<p>Unit issues can be accepted if properly analyzed in relation to mole fraction. (C)</p> Signup and view all the answers

What is the correct representation of chemical exergy?

<p>e^ch^ (B)</p> Signup and view all the answers

Which terms are used to calculate total exergy?

<p>Thermomechanical and chemical exergy (D)</p> Signup and view all the answers

In the chemical formula CaHbOc, what do the subscripts a, b, and c represent?

<p>Number of molecules of carbon, hydrogen, and oxygen (D)</p> Signup and view all the answers

How can chemical exergy for a fuel be evaluated if specific properties are unknown?

<p>By splitting entropy into absolute and partial pressure components (C)</p> Signup and view all the answers

What does the y^e^ term represent in the context of chemical exergy?

<p>Mole fraction of the component in the environment (A)</p> Signup and view all the answers

What is the role of the Gibbs function in simplifying chemical exergy calculations?

<p>It allows for evaluation at standard conditions (P0 and T0) (D)</p> Signup and view all the answers

Which model is recommended for determining the standard chemical exergy for hydrocarbons in this course?

<p>Model II (D)</p> Signup and view all the answers

What is referred to as absolute entropy?

<p>Entropy relative to a specific datum (B)</p> Signup and view all the answers

Which equation correctly describes the total mixture entropy for an ideal gas mixture?

<p>s_total = s_1 + s_2 + ... + s_n (B)</p> Signup and view all the answers

What is the significance of evaluating entropy in combustion processes?

<p>To analyze the energetic efficiency of burning fuels (D)</p> Signup and view all the answers

Which statement accurately describes the relationship between mole fraction and partial pressure in a gas mixture?

<p>Partial pressure equals mole fraction multiplied by total pressure (C)</p> Signup and view all the answers

When calculating entropy production for a fuel at steady state, which variable represents the fuel?

<p>F (C)</p> Signup and view all the answers

What characterizes the absolute entropy tables used for ideal gases?

<p>They provide absolute values at any temperature and pressure (D)</p> Signup and view all the answers

At what temperatures and pressures are the reactants considered for the combustion of liquid octane in the example provided?

<p>25°C and 1 atm (C)</p> Signup and view all the answers

Entropy analysis in combustion is primarily concerned with which aspect?

<p>Understanding entropy production and exergy (D)</p> Signup and view all the answers

During combustion analysis, which condition is maintained as indicated in the example?

<p>There is no heat transfer to the surroundings (D)</p> Signup and view all the answers

Which principle does the equation for partial pressure in a gas mixture rely on?

<p>Dalton's law of partial pressures (D)</p> Signup and view all the answers

Flashcards

Gibbs Function

A thermodynamic property that measures the available energy to do useful work at constant temperature and pressure.

Gibbs Function of Formation

The change in Gibbs function for the reaction when a compound is formed from its elements in their standard state.

Gibbs Function in an Ideal Gas Mixture

In an ideal gas mixture, the Gibbs function of each component is calculated at its partial pressure and the mixture temperature.

Chemical Exergy

A measure of the potential to do useful work due to differences in composition between a system and its environment.

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Exergy Reference Environment

A reference environment with a defined composition and state, used for comparing exergy values.

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Chemical Exergy of Substance CaHbOc

The maximum theoretical work that can be obtained from a mole of a substance reacting completely to form CO2 and H2O.

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Standard Chemical Exergy

A tabulated value that represents the amount of energy available during a chemical reaction, based on the difference in Gibbs functions between reactants and products.

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Chemical Formula CaHbOc

A specific chemical compound or mixture where a, b, and c represent the number of carbon, hydrogen, and oxygen atoms, respectively.

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Reactants for Chemical Exergy Calculation

The amount of oxygen needed to completely burn a fuel to CO2 and H2O.

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Mole Fraction in Environment (ye)

The mole fraction of a component in the environment, which is used as a reference state for calculating exergy.

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Absolute Entropy

The entropy of a substance at a specific temperature and a reference pressure of 1 atm. It is often used as a baseline for entropy calculations.

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Partial Pressure

The pressure exerted by a component in a mixture of gases. It depends on the mole fraction of the component and the total pressure.

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Mole Fraction

The ratio of the moles of a component to the total number of moles in a mixture.

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Entropy Production

The measure of the total entropy change in a process. It represents the irreversibility of the process and is always greater than or equal to zero.

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Exergy

A thermodynamic property, also known as 'available work,' is the maximum work that can be obtained from a system as it reaches equilibrium with its surroundings.

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Exergetic Efficiency

A measure of how efficiently a process converts energy into useful work. It is calculated by dividing the useful work output by the total energy input.

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Steady State

A process that occurs without any external influence or energy input. This implies that the process is reversible and no entropy is produced.

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Burning

A chemical reaction that involves the combustion of a fuel with air, typically used to generate energy.

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Complete Combustion

The state where a reaction has been fully completed. All the reactants have been converted into products.

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Theoretical Air

A coefficient that represents the amount of air supplied to a combustion process relative to the stoichiometric amount required for complete combustion.

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Study Notes

Combustion Supplemental Notes

  • Combustion is the rapid oxidation of combustible elements in fuels, releasing energy in the form of combustion products.
  • Three major combustible elements are carbon (C), hydrogen (H), and sulfur (S). Common examples include methane, ethane, and octane.
  • Complete combustion occurs when all carbon is burned to carbon dioxide (CO2), all hydrogen is burned to water (H2O), and all sulfur is burned to sulfur dioxide (SO2). Incomplete combustion results in other products like CO, NOx, and other impurities.
  • Oxygen is necessary for combustion, and air is commonly used as the oxygen source.

Modeling Combustion Air

  • Dry air is often modeled as 21% oxygen (O2) and 79% nitrogen (N2) by moles.
  • For every mole of oxygen used, air also supplies 3.76 moles of nitrogen.
  • Nitrogen is considered inert in most combustion calculations.
  • Molecular weight of dry air is approximately 28.97.
  • In situations with moist air, water vapor content must be accounted for in combustion calculations.

Air-Fuel Ratio

  • Air-fuel ratio (AF) is the ratio of the amount of air to the amount of fuel. It can be expressed on a molar or mass basis.
  • Fuel-air ratio is the reciprocal of air-fuel ratio.

Step 1: Balancing Reactions

  • Example: Complete combustion of methane (CH4) with oxygen (O2) yields carbon dioxide (CO2) and water (H2O).
  • Balancing the reaction involves conserving the atoms of carbon, hydrogen, and oxygen.
  • The balanced reaction equation for methane combustion is CH4 + 2O2 → CO2 + 2H2O

Theoretical Amount of Air

  • Theoretical air is the minimum amount of air required to completely oxidize all the carbon, hydrogen, and sulfur in the fuel.
  • Products of complete combustion with theoretical air are CO2, H2O, SO2, and nitrogen from the air.
  • Often combustion uses more or less than the theoretical amount of air, commonly expressed as a percentage of theoretical air or as percentage of excess air (or deficiency).

Dry Product Analysis

  • Often, combustion product analysis is reported as a dry basis. This means water is removed. Given this, percentages are for all products except water.
  • Example problems demonstrate determining the balanced chemical reaction from the dry product analysis percentages.

Step 2: First Law Energy Balance of Reacting Systems

  • Evaluate heat transfer rates during combustion reactions.
  • Consider the enthalpy of the reactants and products for the energy balance.
  • Calculations take into account kinetic and potential energy and use the ideal gas model.

Step 3: Evaluating Properties

  • Combustion reactions are often modeled as ideal gas mixtures. Ideal gas mixture principles are used in determining internal energy, enthalpy, and entropy.
  • Enthalpy of formation and absolute entropy are key concepts used when evaluating properties for reacting systems.

Heating Values of Hydrocarbon Fuels

  • The heating value (HV) is the energy released when a fuel is burned completely, with the reactants and products at the same temperature and pressure.
  • Higher Heating Value (HHV) accounts for water being in liquid form.
  • Lower Heating Value (LHV) accounts for water being released as a vapor.

Adiabatic Flame Temperature

  • Adiabatic flame temperature is the temperature of the products of a combustion reaction under adiabatic conditions (no heat transfer).
  • Theoretical air amount is used to determine the maximum adiabatic flame temperature.
  • Equations and EES problem-solving methods are shown to determine adiabatic flame temperature.

Chemical Exergy

  • Chemical exergy measures how far a system's composition is from the exergy reference environment.
  • Total exergy is the sum of thermochemical and chemical exergy in a reacting system.
  • Equations are provided for calculating chemical exergy using the Gibbs function.

Absolute Entropy and the Third Law of Thermodynamics

  • The third law of thermodynamics states that the entropy of a pure crystalline substance is zero at absolute zero temperature.
  • Absolute entropy is the entropy of a substance relative to the entropy of a pure crystalline substance at absolute zero. This value is used for calculations.

Entropy Balance

  • Entropy balance is used to determine entropy production during combustion reactions on a per mole basis.
  • Equations are provided, along with example problems to guide the calculations.

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Related Documents

Combustion Supplemental Notes PDF

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