1 Chemical thermodynamics Flashcards
What does chemical thermodynamics deal with?
The description of the equilibrium states of reacting multicomponent systems
What are global performance parameters?
- heat
- power output
How can global performance parameters be estimated?
By assuming thermodynamic equilibrium
Which effect does the big amount of nitrogen have in the air?
It reduces
the combustion temperature
because of the expenditure of thermal energy used to heat it up during the course of burning.
Which measures can be taken for applications requiring intense burning
use of
- oxygen enriched air
- pure oxygen
Representative components of gaseous fuels
- hydrogen (H2)
- carbon monoxide (CO)
- light hydrocarbons (HC)
Representative components of liquid fuels
heavier hydrocarbons and alcohols
Representative components of solid fuels
- carbon
- coal
- wood
- metals
What does the combustion intensity between a fuel and an oxidizier depend on?
It depends on their relative concentrations
When is a reaction fuel lean?
< 1
Practical HC reactants (not necessarily gaseous)
- alkanes (paraffins)
- alkenes (olefins)
- acetylenes (alkynes)
- diolefins (two CC)
- napththenes (cycloparaffins or cyclanes)
- aromatics
- alcohols
Characteristics of napththenes
- C(m)H(2m)
- single-bond
- saturated
- closed rings
Characteristics of aromatics
- C(m)H(2m-6)
- single-bond
- double-bond alternate in position
Characteristics of alcohols
- R-OH
- C(m)H(2m+1)OH
What does the first law of thermodynamics state?
For a closed system, the heat added to the system is used to increase its intenral energy and to perform a certain amount of work
How many modes of reversible work exist?
One : pdV
How can the state of a single-component thermodynamic system be characterized?
By two independent variables: S and V (second law of thermodynamics)
How do you deal with multicomponent systems?
The same species in different phases is treated as different thermodynamic species by considering phase transition as a chemical reaction
Does the presence of inerts in the system affect the equilibrium constant?
No, but it needs to be accounted for, when the partial pressures are related to either the molar fraction or the mass fraction.
How do you consider solid carbon in the product for HC/air mixtures?
- Assume C(s) is present, then check the sign of C(s)
- Assume there is no C(s), check partial pressure of C(g) If the calculation shows that the partial pressure of the gaseous carbon exceeds the equilibrium vapor pressur of carbon, then condensation and solid carbon must exist.
Which indicator lets you estimate if solid carbon is likely to be formed?
Equivalence ratio.
For <1, there exists enough oxygen for complete conversion, implying that carbon, or soot emission, usually observed only for rich mixtrues.
What does the composition depend on in a fuel lean composition according to major-species- model?
It does NOT depend on T, P
It just depends on equivalence ratio
What does the composition depend on in a fuel rich composition according to major-species- model?
It depends on T, P and equivalence ratio.
Which equilibrium composition calculations exist?
- Major-minor species model
- Minor species model
- Computer Code
Which are major species?
- CO2
- H2O
- H2
- O2
- CO
What do we assume for fuel-lean mixtures?
We assume the complete conversion of
- C to CO2
- H to H2O
- with the excess oxygen remaining as O2
What are minor species?
- OH
- O
- H
- NO
- N
What’s a minor specie for lean mixtures?
CO
What’s a minor specie for rich mixtures?
O2
What do we assume for minor species calculations?
We assume that the concentrations of the major species remain unaffected by the production of the minor specie
Therefore they remain at their respective levels determined in the major specie step.
When is the dissociation of H2O and CO2 not negligible?
- when psi about 1
- when T > 2000K
Which minor specie is most likely in fuel lean combustion?
NO
Which minor specie is most likely in fuel rich combustion?
O2
What takes place during reactions?
Exchanges in chemical and thermal energy
What is the heat of formation?
It is the amount of heat needed to form one mole of the substance
from its elements in their standard states,
with the reaction taking place in a closed system,
with the initial and final temperature being the same.
What does it mean if the heat of formation is positive?
Heat is absorbed by the system
-> endothermic
What does it mean if the heat of formation is negative?
Heat is released by the system
-> exothermic
What’s the heat of combustion?
A special class of the heat of reaction
It is the amount of heat release when 1 mole of fuel in its standard state
is completely reacted with oxygen to form H2O, CO2 and N2
What’s the lower heating value of a fuel?
Heat of combustion with water present in gaseous state
What’s the higher heating value of a fuel?
Heat of combustion with water present in liquid state
What’s the quantitative difference between higher and lower heating value
10,52 kcal/mole for every mole of H2O produced
How can you obtain the heat of reaction without the heat of formation?
You can estimate the value of the heat of formation from the bond energies between the atomic constituents of the reactant and product molecules
Which types of sensible energies do you have?
- translational
- rotational
- vibrational
- electronic
What does the excitation of the vibrational modes depend on?
On temperature: increasing with increasing temperature
What does a large heat capacity imply
It implies the activation of more modes of excitation of the molecules
This the given amount of energy has to be distributed over more excitation modes leading to less energy for each mode and thereby a smaller temperature increase
What does the composition of the major species in fuel-lean mixtures depend on?
On nothing.
Since it is completely specified through stoichiometry, it does not depend on the temperature and pressure of the mixture.
Which are the most important factors influencing the adiabatic temperature?
- The equivalence ratio
- the C/H ratio
Why do we have a peak of the adiabatic temperature for an equivalence ratio of 1?
The adiabatic temperature decreases steadily as the mixture becomes either leaner or richer because of the need to heat up the excess reactants for off-stoichiometric conditions.
Why does the C/H ratio have an influence of the adiabatic temperature?
- Species with large C/H ratios often contain more double and triple C-C bonds which can hold more potential energy then the single bond
What can reduce dissociation?
Increased pressure
Therefore the adiabatic temperature rises with pressure
Does dissociation have an influence on the heat capacity?
No
It increases monotonically from lean to rich mixtures.
This is caused by the increasing number of moles of product species formed per unit mass of the mixture reacted , especially those of H2 and CO, as the mixture becomes progressively richer.
The heat capacity increases from lean to rich.
What does it imply for the adiabatic flame temperature?
It tends to cause the peak of the AFT shift to the lean side.
What’s the cause of the rich shifting?
Dissociation of CO2 and H2O
The Arrhenius law is valid to
All kinds of reactions
What is special about coupling functions?
Temperature and species concentration are nondimensional
