15.2 Theoretical and actual combustion processes
4 min read•july 30, 2024
Combustion processes are crucial in thermodynamics, powering everything from cars to power plants. assumes perfect fuel burning, while deals with real-world inefficiencies and incomplete reactions.
Understanding the differences between ideal and non-ideal combustion helps engineers design better engines and reduce emissions. Air-fuel ratios and efficiency calculations are key tools for optimizing combustion systems and improving overall performance.
Theoretical vs Actual Combustion
Ideal vs Non-ideal Processes
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- Theoretical combustion is an ideal process that assumes complete combustion of the fuel with the exact stoichiometric amount of air required
- Results in only carbon dioxide, water vapor, and nitrogen as products
- Represents the most efficient and clean combustion scenario (ideal case)
- Actual combustion is a non-ideal process that occurs in real-world applications
- Combustion is often incomplete due to insufficient mixing, short residence times, or other factors
- Results in additional products like unburned hydrocarbons (methane), carbon monoxide, and other pollutants, in addition to carbon dioxide, water vapor, and nitrogen
- Represents the practical limitations and inefficiencies of combustion systems (real case)
Air-Fuel Ratio and Efficiency
- Actual combustion processes may involve excess air () or insufficient air ()
- Lean combustion (excess air) leads to lower flame temperatures and reduced NOx emissions but may cause instability and
- Rich combustion (insufficient air) leads to higher flame temperatures, increased CO and soot formation, and reduced efficiency
- (exact air requirement) provides the optimal balance between efficiency and emissions
- The efficiency of actual combustion processes is typically lower than that of theoretical combustion
- Heat losses to the surroundings through conduction, convection, and radiation
- Incomplete combustion due to poor mixing, insufficient residence time, or quenching
- Other irreversibilities such as friction, turbulence, and non-equilibrium effects
- Efficiency can be improved through better combustion chamber design, fuel atomization, and air-fuel mixing
Air-Fuel Ratio Calculations
Air-Fuel Ratio (AFR) and Stoichiometric AFR
- The (AFR) is the ratio of the mass of air to the mass of fuel in a combustion process
- Expressed as a dimensionless quantity (15) or as a mass ratio (15:1 or 15 kg air per kg fuel)
- Determines the overall composition of the combustion mixture and affects the combustion characteristics
- The (AFR_stoich) is the exact amount of air required for complete combustion of a given fuel
- Determined from the balanced chemical equation of the combustion reaction
- Represents the ideal air-fuel ratio for complete combustion without excess air or unburned fuel
- Example: For the combustion of methane (CH4) in air, the stoichiometric AFR is approximately 17.2:1
Equivalence Ratio (φ)
- The (φ) is the ratio of the actual fuel-to-air ratio to the stoichiometric fuel-to-air ratio
- Defined as: φ = (Fuel/Air)_actual / (Fuel/Air)_stoichiometric
- Alternatively, can be expressed as: φ = AFR_stoich / AFR_actual
- φ < 1 indicates lean combustion (excess air), φ > 1 indicates rich combustion (excess fuel), and φ = 1 represents stoichiometric combustion
- Example: For an actual AFR of 14.7:1 and a stoichiometric AFR of 15.5:1, the equivalence ratio is φ = 15.5 / 14.7 = 1.05 (slightly rich combustion)
- The air-fuel ratio and equivalence ratio are essential parameters in designing and optimizing combustion systems
- Affect the , emissions, flame stability, and overall performance
- Can be controlled through fuel metering, air flow regulation, and combustion chamber design
- Often monitored and adjusted in real-time using sensors and feedback control systems
Combustion Products Determination
Stoichiometry and Balanced Chemical Equations
- Stoichiometry is used to determine the quantities of reactants and products in a balanced chemical reaction
- Based on the conservation of mass and the molar ratios of the species involved
- Requires a balanced chemical equation that accurately represents the combustion reaction
- Example: For the combustion of propane (C3H8) in air, the balanced equation is: C3H8 + 5(O2 + 3.76N2) → 3CO2 + 4H2O + 18.8N2
- The general steps for determining the products of combustion using stoichiometry are:
- Write a balanced chemical equation for the combustion reaction, considering the fuel composition and the oxidizer (typically air)
- Identify the given information, such as the mass or moles of fuel, the air-fuel ratio, or the equivalence ratio
- Calculate the moles of fuel and air using the given information and the molar masses of the species
- Use the molar ratios from the balanced equation to determine the moles of each product
- Convert the moles of products to mass or volume, if required, using molar masses or the ideal gas law
Complete vs Incomplete Combustion Products
- The products of complete combustion of hydrocarbons in air are primarily carbon dioxide (CO2), water vapor (H2O), and nitrogen (N2)
- Traces of other compounds like argon (Ar) and oxygen (O2) may also be present due to their presence in air
- Complete combustion requires sufficient oxygen, mixing, and residence time to fully oxidize the fuel
- In the case of incomplete combustion or fuel-rich conditions, the products may also include:
- Carbon monoxide (CO) due to partial oxidation of the fuel
- Unburned hydrocarbons (UHCs) such as methane, ethylene, and acetylene
- Hydrogen (H2) from the decomposition of the fuel molecules
- Soot (solid carbon particles) formed through fuel pyrolysis and agglomeration
- These products are generally undesirable due to their negative impact on efficiency and the environment
- When dealing with combustion reactions involving non-hydrocarbon fuels or oxidizers other than air, the stoichiometry must account for the specific elements present and their respective molar ratios
- Example: The combustion of hydrogen sulfide (H2S) in pure oxygen produces water vapor and sulfur dioxide (SO2): 2H2S + 3O2 → 2H2O + 2SO2
Key Terms to Review (22)
Actual combustion: Actual combustion refers to the real-world process of burning fuel, where the combustion occurs in a system under specific conditions and often deviates from ideal scenarios due to various factors such as incomplete combustion, heat losses, and excess air. This term highlights the practical aspects of combustion that can lead to differences in performance compared to theoretical predictions, which assume perfect conditions.
Adiabatic flame temperature: Adiabatic flame temperature is the maximum temperature that can be achieved during a combustion process when no heat is lost to the surroundings. This temperature is significant because it reflects the efficiency of fuel combustion under ideal conditions, without any heat transfer or losses. Understanding this concept helps in analyzing both theoretical combustion processes and actual reacting systems, and it provides insights into how different variables affect flame behavior and energy output.
Air-fuel ratio: The air-fuel ratio is the proportion of air to fuel in a combustion process, expressed as a mass or volume ratio. This ratio is critical in determining combustion efficiency, emissions, and overall performance of combustion systems. It influences the completeness of combustion, with an optimal ratio leading to efficient energy release while minimizing unburned fuel and harmful emissions.
Combustion efficiency: Combustion efficiency refers to the measure of how effectively a fuel is converted into usable energy during the combustion process. It indicates the percentage of fuel energy that is transformed into useful work or heat, and the efficiency can vary between theoretical and actual combustion processes. Understanding combustion efficiency is crucial for optimizing performance, minimizing emissions, and assessing the adiabatic flame temperature, which represents the maximum temperature achievable in a combustion reaction without heat loss.
Energy balance equation: The energy balance equation is a fundamental principle that states that the energy entering a system must equal the energy leaving the system plus any change in the energy stored within that system. This concept is crucial for analyzing various processes and systems, enabling the calculation of energy transformations, efficiencies, and performance metrics in engineering applications.
Enthalpy of Formation: Enthalpy of formation is the change in enthalpy when one mole of a compound is formed from its elements in their standard states. It serves as a crucial concept in thermodynamics, helping to quantify the energy changes during chemical reactions, especially combustion processes. Understanding this term is key to analyzing energy balances in reacting systems and determining the adiabatic flame temperatures resulting from combustion.
Equivalence ratio: The equivalence ratio is a dimensionless number that compares the actual fuel-to-air ratio in a combustion process to the stoichiometric fuel-to-air ratio required for complete combustion. It indicates whether the mixture is rich (greater than 1), stoichiometric (equal to 1), or lean (less than 1). Understanding the equivalence ratio is crucial for analyzing both theoretical and actual combustion processes, as it influences combustion efficiency, emissions, and the formation of pollutants.
First Law of Thermodynamics: The First Law of Thermodynamics states that energy cannot be created or destroyed, only transformed from one form to another, which means the total energy of an isolated system remains constant. This principle underlies various processes, cycles, and energy interactions that involve heat, work, and mass transfer in different systems.
Flame propagation: Flame propagation refers to the process by which a flame moves through a combustible mixture of gases, vapors, or solids, driven by the heat generated during combustion. This phenomenon is crucial in understanding combustion dynamics and the efficiency of combustion systems, as it influences how quickly and evenly fuel burns in various applications.
Incomplete combustion: Incomplete combustion occurs when there is not enough oxygen present to allow for the complete oxidation of a fuel, leading to the production of carbon monoxide, soot, or other hydrocarbons instead of just carbon dioxide and water. This process significantly affects energy efficiency and environmental emissions, making it crucial to understand in the context of fuels and combustion as well as theoretical versus actual combustion processes.
Lean combustion: Lean combustion refers to a combustion process in which the fuel-to-air ratio is lower than the stoichiometric ratio, meaning there is more air than what is needed for complete combustion of the fuel. This results in a more efficient burning process, leading to reduced emissions and improved fuel economy, but it may also cause increased levels of nitrogen oxides (NOx) due to higher combustion temperatures.
Pre-flame phase: The pre-flame phase refers to the initial stage of combustion where fuel and oxidizer are prepared for ignition but have not yet reacted. This phase involves the mixing and heating of the reactants, which is crucial for achieving the proper conditions necessary for combustion to initiate. Understanding this phase is essential as it influences the efficiency and stability of the subsequent combustion processes.
Rich combustion: Rich combustion refers to a combustion process in which there is an excess of fuel compared to the amount of oxidizer present, leading to a higher fuel-to-air ratio. This condition often results in incomplete combustion, producing more unburned hydrocarbons and particulate matter, which can negatively affect emissions and overall efficiency. Rich combustion can be relevant in various applications, including internal combustion engines and industrial burners.
Robert H. Perry: Robert H. Perry is a notable figure in the field of chemical engineering, particularly recognized for his contributions to thermodynamics and combustion processes. His work emphasizes the theoretical and actual combustion processes that are crucial for understanding energy systems, fuel efficiency, and environmental impacts in engineering applications.
Sadi Carnot: Sadi Carnot was a French physicist and engineer who is best known for his foundational work in thermodynamics, particularly the concept of the Carnot cycle, which describes the idealized heat engine. His theories introduced important principles regarding the efficiency of engines and the limits of energy conversion, connecting heat transfer processes to mechanical work. Carnot's insights laid the groundwork for understanding isentropic processes and have implications for real combustion systems.
Second Law of Thermodynamics: The Second Law of Thermodynamics states that the total entropy of an isolated system can never decrease over time, and it tends to increase, leading to the concept that energy transformations are not 100% efficient. This law establishes the directionality of processes, implying that certain processes are irreversible and energy has a quality that degrades over time, connecting tightly to concepts of heat transfer, work, and system analysis.
Specific heat at constant pressure (cp): Specific heat at constant pressure, denoted as $$c_p$$, is the amount of heat required to raise the temperature of a unit mass of a substance by one degree Celsius while maintaining constant pressure. This property is crucial in understanding how substances absorb heat during processes like combustion, where temperature changes can significantly impact performance and efficiency.
Specific heat at constant volume (c_v): Specific heat at constant volume (c_v) is the amount of heat required to raise the temperature of a unit mass of a substance by one degree Celsius while keeping the volume constant. This property is critical in analyzing energy transfers and transformations during combustion processes, where controlling the volume can influence efficiency and reaction rates.
Stoichiometric air-fuel ratio: The stoichiometric air-fuel ratio is the ideal ratio of air to fuel needed for complete combustion of a fuel in a combustion process, ensuring that all the fuel reacts with oxygen without any excess of either reactant. This concept is crucial because it determines the efficiency and emissions of combustion systems, helping in the design and optimization of engines and burners. Achieving this ratio means maximizing energy output while minimizing pollutants, which is vital in both theoretical models and actual combustion processes.
Stoichiometric combustion: Stoichiometric combustion refers to the ideal reaction of a fuel with an oxidizer, typically oxygen, where the amounts of both reactants are perfectly balanced to achieve complete combustion with no excess reactants left over. This concept is crucial for understanding theoretical and actual combustion processes, as it helps in calculating the maximum energy output and establishing the conditions for optimal combustion efficiency. It also plays a significant role in determining the adiabatic flame temperature, which is the temperature that would be achieved if all the heat released during combustion were used to raise the temperature of the products without any losses.
Theoretical combustion: Theoretical combustion refers to the idealized process of burning fuel in which all reactants are completely converted into products without any losses or inefficiencies. This concept serves as a benchmark to assess actual combustion processes, highlighting discrepancies due to factors such as incomplete combustion, heat losses, and pollutants. Understanding theoretical combustion is crucial for optimizing fuel efficiency and minimizing emissions in real-world applications.
Thermal efficiency: Thermal efficiency is a measure of how well an energy conversion system, such as a heat engine, converts heat energy into useful work. It is defined as the ratio of the useful work output to the heat input, typically expressed as a percentage. This concept is crucial for evaluating and optimizing the performance of various thermodynamic cycles and systems.