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Chapter 2. Fundamental Concepts in Understanding Bioenergy and Biobased Products Engineering Thermodynamics. Introduction. •Thermodynamics essential to designing processing systems for biorenewable resources ( Net energy output must be positive!!! ) •Fundamental concepts include
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Chapter 2. Fundamental Concepts in Understanding Bioenergy and Biobased Products
Engineering Thermodynamics
•Thermodynamics essential to designing processing systems for biorenewable resources (Net energy output must be positive!!!)
•Fundamental concepts include
–Mass balances
–Energy balances
•These lectures not a substitute for a course in engineering thermodynamics
•Isolated system –neither mass nor energy enters the system
•Closed system –mass does not enter or leave the system (no restriction on energy flow)
•Open system –both mass and energy can flow through the system
Mass Balances for Combustion Processes
Mass Balances for Combustion Processes (Continues)
Example
Consider a mixture of one mole of ethane (C2H6) and one mole of oxygen (O2).
F/O ratio of this mixture based on the mass of fuel and O2 is:
F/O ratio of this mixture based on the number of moles of fuel and O2 is:
0.5
2
To calculate the equivalence ratio, we need to first write out the stoichiometric reaction of ethane and oxygen
Based on Mass:
Based on Mole:
(F/O)actual
0.938
0.5
(F/O)stoichiometric
30/112 = 0.268
1/(7) = 0.143
Equivalence
Ratio
3.5
3.5
Mass Balances for Combustion Processes (Continues)
Actual Air
X 100
Stoichiometric Air
Actual Air – Stoichiometric Air
X 100
Stoichiometric Air
Energy Balance for Open System
*he or hi = specific enthalpy (energy / mass)
*he or hi = specific molar enthalpy (energy / mole)
*He or Hi = enthalpy (energy)
Energy Balance for Open System
.
.

(Based on Mass)
S
S
mph
mrh

p
r
p
r
.
.
(Based on Mole)
S
S
nph
nrh

p
r
p
r
Hp
Hr
Energy Balance for Open System
Example:
Let us assume that 1 kmole/hr of biogas is produced by anaerobic digestion of animal waste consists of 60% of CH4 and 40% of CO2 (molar basis). The biogas reacts with 1.2 kmol/hr of O2 to form CO2 and H2O (no other products).
Biogas + O2
T = To = 298K
Q
T = T2 = 1500K
CO2 + H2O
0.6 CH4 + 0.4 CO2 + 1.2 O2 CO2 + 1.2 H2O
We want to calculate Q under steady state condition for this example with following additional info.
o
The standard enthalpy of reaction (hR) is 890,330 kJ/kmol of CH4 at 298K.
Step #1: Do Energy Balance
=
H

=
Reactants
H
Products
H
HR (To)
T
To = 298K
T2 = 1500K
HR(To)
H =
+
H = (890,00)*(0.6) + [1*(71,0789,364) + (1.2)*(57,9999,904)] = 414,770 kJ/hr
h
p
(
r
(
p
r
Example:
Calculate the standard heat of reaction for the dehydrogenation of ethane:
C2H6 C2H4 +H2
• Most biomass fuels are not well characterized in terms of their chemical constituents
–Often simpler to perform calorimetric tests on biomass fuels to
determine enthalpy of reaction
• Every energy conversion process can be characterized by its thermodynamic efficiency
A B
If G > 0, then the reaction is not spontaneous.
If G < 0, then the reaction is spontaneous.
ln