enthalpy of formation and combustion

19 Pages·2014·1.67 MB·English

Akash
Thahrani
K-13ME-05
MUET,SZAB
KKhhaaiirrppuurr
FIRST-LAW ANALYSIS OF REACTING SYSTEMS
1st law of thermodynamics:
Reacting systems: the system in which chemical reaction take place
Steady-flow devices: operate for long periods of time under the same conditions.
Enthalpy of reaction or Enthalpy of formation:
Enthalpy of Combustion:
“ It can be viewed as the enthalpy of a
It is the difference between the substance at a specified state due to its
enthalpy of the products at a specified chemical composition. “
state and the enthalpy of the reactants
at the same state for a complete
reaction.
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Enthalpy of a chemical component
The enthalpy of a chemical component at a
specified state is the sum of
the enthalpy of the component at 25°C,
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component relative to 25 °C, 1 atm.
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FIRST-LAW ANALYSIS OF REACTING SYSTEMS
Steady-Flow Systems
 When the changes in kinetic and potential energies are negligible,
 The steady-flow energy balance relation for a chemically reacting steady-
flow system can be written as:
 Taking heat transfer to the system to be positive
 and work done by the system to be positive
 The energy balance relation just discussed can be expressed more compactly as:
_
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Most steady-flow combustion processes :
 do not involve any work interactions.
 involves heat output but no heat input
It expresses that :
“The heat output during a combustion process is simply the difference between the
energy of the reactants entering and the energy of the products leaving the
combustion chamber.”
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We note that
 all the hydrogen in the fuel burns to H2O
 but 10percent of the carbon burns incompletely and forms CO.
 Also, the fuel is burned with excess air and thus there is some free O2 in
the product gases.
Theoretical amount of air:
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Then the balanced equation for the actual combustion process with 50 percent
excess air and some CO in the products becomes
(a) The air–fuel ratio for this combustion process is:
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(b) The heat transfer for this steady-flow combustion process is determined
from the steady-flow energy balance:
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