Thermal Energy Recovery
Ladokcode:
The exam is given to:
5.0 ECTS
42RT05
Resource Recovery
ExamCode:
Date of exam:
Time:
2016-01-11
9:00-13:00
Means of assistance:
English Dictionary, Calculator, Steam tables
Total amount of point on exam:
Requirements for grading:
Grade F: Less than 17 points
Grade FX: Minimum 17 points
Grade E: Minimum 20 points
Grade D: Minimum 25 points
Grade C: Minimum 30 points
Grade B: Minimum 35 points
Grade A: Minimum 40 points
50
Additional information:
Next re-exam date:
The marking period is, for the most part, 15 working days, otherwise it’s the following date:
Important! Do not forget to write the ExamCode on each paper you hand in.
Good Luck!
Examiner:
Phone number:
Tobias Richards
073-2305964
Part I Combustion (16)
1. A fuel is analysed in a laboratory. The gross calorific value (Hs) is determined to 10 MJ/kg, the
moisture content is 50 % of the fuel, the content of hydrogen is 3 % of the fuel and the content of ash
is 0.75 % of the fuel. Calculate the net calorific value (Hi) of the fuel.
(5p)
2. One of the boilers at the waste fired combined heat and power plant Händelöverket in Norrköping is
operated with an excess air ratio so that the oxygen content (O2) in the flue gas is 3 % on dry gas, i.e.
neglecting the moisture in the gas. The concentration of carbon dioxide (CO2) is 18% and the
nitrogen oxide (NO) is 200 mg/mn3. However, the combustion is inefficient and generates too much
carbon monoxide (CO). Therefore, the excess air ratio is increased so that O2 becomes 6 % on dry
gas. Does it affect the CO2 concentration and the NOx concentration in the flue gas? If so, which is
the new CO2 concentration?
(4p)
3. There are a number of different sources of automotive biofuels, e.g. ethanol made from grain, biogas
generated from gasification of wood and biogas produced from manure. Which of these three is the
best regarding the emission of greenhouse gases (CO2 equivalents) Explain why!
(4p)
4. The following fuels have different calorific values: Coal, wood, peat, brown coal and anthracite.
Arrange them in decreasing net calorific value.
(3p)
Part II Inorganic reactions (9 p)
5. Ashes are rest products from combustion and contain different material entering the boiler as a part
of the fuel or as contaminants.
a. What materials are found in bottom slagg from grate boilers combusting solid waste? Give 4
(2p)
examples.
b.
Give 4 examples on elements that form compounds with low melting point that can cause
deposit formation and increased corrosion.
(2p)
6. When analysing the combustion environment in different combustion scenarios various methods are
used. Often the deposit formation is monitored to give some answers.
a. What answers can you get by a deposit formation analyse using a deposit probe with
collection rings?
(3p)
b. There are four mechanisms causing deposit formation, which?
(2p)
Part III Gasification and thermal treatment (15 p)
7. What steps occur during gasification? Explain for each step the main reaction/reactions and list the
main products and the remaining materials as well as an estimation of how much material that is left
as solid after each step when a typical woody biomass is gasified.
(5p)
8. In a system, char is gasified with CO2 (the reaction equilibrium constant can be estimated as k =
4.47·109·e(-177000/RT)) according to C+CO2  2 CO
a. Write the expression for the equilibrium constant based on concentrations
(1p)
b. To get a high conversion of the char would it demand more CO2 at a low or at a high
temperature?
(2p)
1
c. Would it be easier or more difficult to perform the gasification at higher pressure? Motivate.
(2p)
9. There are different types of gasifiers.
a. Describe how an indirect heated fluidized bed gasifier works. Explain the benefits and
drawbacks.
(3p)
b. What would be the most likely oxidizing media?
(1p)
c. How can heat be transferred to the reactor (endothermic reaction)
(1p)
Part IV Materials, erosion and corrosion (10p)
10.
Material properties are important in thermal systems due to the harsh environment. In the
course we discussed different groups of materials and their characteristics. Describe what properties
which are characteristic for composites.
(3p)
ɛ [MPa]
0
10
20
30
40
50
60
70 t [min]
11. A creep diagram is showed in the figure above. What can you see from this diagram? When begins
and ends the three stages of creep and what is happening?
(3p)
12. An Ellinghm/Richardsson diagram is handy when to determine how a metal or metal-alloy will
oxidise. An Inconel Fe21Cr32Ni Super alloy is used in the superheater package in a boiler. The
surface temperature of the tubes is 700°C and the partial pressure of oxygen is 10-20atm.
a. Describe the formed oxide layer on the surface of these super heater tubes.
(2p)
b. Describe what catastrophic oxidation is and why it occurs
(2p)
Good luck!
2
Stress, σ= Eε
σ= F/Ao
Strain, ε = (L-Lo)/Lo = ∆L/Lo
%EL = (Lf-Lo)/Lo)*100
Lf = Lo+ εf Lo
(1 N/mm2 = 106 N/m2 = 1 MPa)
Oxidation kinetiks:
Linear oxidation
x = k1t + D
Parabolic oxidation
dx k p
=
⇒ x = 2k p t + C
x
dt
Logarithmic oxidation
x = kl log t + A
3
4