Thermal Energy Recovery
5.0 ECTS
Ladokcode: 42RT05The exam is given to: Msc Resource Recovery, KMAREC17h ExamCode:
Date of exam: 2018-01-08 Time: 14:00-18:00 Means of assistance:
English Dictionary, Calculator, Steam tables
Total amount of point on exam: 50 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
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.
Part I Combustion (16)
1. In a steam boiler of Clausius-Rankine type the working medium is water/steam. Describe the loop this medium passes through in the boiler plant and explain briefly its main components. (5p)
2. Mölndal Energy AB plans to build one new boiler for combustion of waste. The thermal capacity of the boiler is 20 MW. The fuel has the following composition:
Moisture: 35 % of fuel
Ash: 18.46 % of dry substance C: 47.75 % of dry substance H: 6.29 % of dry substance S: 0.62 % of dry substance O: 25.36 % of dry substance N: 0.75 % of dry substance
The Net calorific value (NCV) is 11.5 MJ/kg. The flue gas contains 6% O2 and has the temperature 150 °C. Mölndal Energy is considering to build also a flue gas condensation unit, to be connected to the boiler. Calculate how much extra heat (MW) that can be recovered from the flue gas if all moisture in the gas is condensed in a flue gas condensing unit. Necessary assumptions made have to
be explained. (9p)
3. There are two different methods to calculate the efficiency of a boiler. Which are these methods?
Explain them and the difference between them. (2p)
Part II Inorganic reactions (9 p)
4. Ash forming compounds often causes trouble in the boiler system during waste combustion.
a. Especially in what/whish aggregation form? (1p)
b. What 4 types of problem do they contribute to? (2p)
5. Different bio- and waste fuels have very different element compositions whish also gives large differences in the amount of ash. List the fuel groups after the their amount of ash starting with the fuel having the smallest ash fraction:
Wood; Solid Waste; Straw; Oil; Sewage Sludge (2p)
6. When deposits are formed on super heater tubes and walls in boilers four mechanisms are identified.
a. Describe how thermophoresis works. (2p)
b. What is characteristic with the deposits formed by thermophoresis. (1p)
Part III Gasification and thermal treatment (15 p)
8. Gasification can be used as an alternative to improve waste treatment (changing from waste combustion).
a. Describe why gasification could improve the waste management. (2p)
b. In several countries such as England, waste combustion is almost prohibited but gasification
is possible. Explain why this could be the case. (2p)
9. There exist several types of gasifiers. One such type is the air blown circulating fluidized bed gasifier.
a. Describe how this system works and sketch the temperatures in the reactor. (2p) b. Will the gasification reaction mainly be endothermic or exothermic? (1p) c. What is the most likely use of the produced gases? Explain your answer. (2p)
d. What are the benefits and drawbacks with a circulating fluidized bed compared to other types
of gasifiers? (2p)
10. One of the products after gasification could be methane (bio-SNG).
a. From the equilibrium calculations, would you use a high or a low temperature to increase the
amount of methane in the outgoing gases? (1p)
b. What is the problem with using this temperature? (1p)
c. What are the benefits of using gasification compared to digestion to produce methane from
waste material? (2p)
Part IV Materials, erosion and corrosion (10p)
11. Most metals form oxides on the surface and the properties of the formed oxide strongly depends on the environment the metal is exposed to.
a. Give 4 properties a oxide need to have to be protective. (2p) b. Describe why the oxide formation of Fe and Cr are so different, se picture below.
12. Here is a Stress-strain diagram for metal X. The specimen of the metal has a diameter of 10 mm and is 90 mm long.
a) Determine the modulus of elasticity. (1p)
b) Determine the elongation when a load of 25 000 N is applied on the specimen. (2p) c) By looking at this diagram, how would you describe metal X? (1p)
d) What is the yield strength, σy, of the metal? (2p)
Tensile Stress, = E = F/Ao Strain, = (L-Lo)/Lo = L/Lo %EL = (Lf-Lo)/Lo)*100 Lf = Lo+ f Lo (1 N/mm2 = 106 N/m2 = 1 MPa) Oxidation kinetiks: Linear oxidation D t k x 1 Parabolic oxidation C t k x x k dt dx p p 2 Logarithmic oxidation A t k x llog
