Exam

Question 1

How is net calorific value defined and how can it be determined?

Answer 1

• The net calorific value Hi (lower heating value, lower calorific value) quantifies the heat, which is released with a full oxidation of a fuel, whereby all combustion products are gaseous.
• The net calorific value can be calculated with the Boie - formula:
Hi = 34,8 C + 93,9 H + 10,5 S + 6,3 N – 10,8 O – 2,5 w [MJ /kg]

Question 2

How is gross calorific value defined and how can it be determined?

Answer 2

• The gross calorific value Hs (higher heating value, higher calorific value) is the net calorific value PLUS the condensation energy of the resulting combustion water (reference temperature: 25°C)
• The gross calorific value is determined experimentally in a bomb
calorimeter
Hs = Hi + 2,44 ⋅ (9 H + w) [MJ/kg]

Question 3

Characteristics of wood biomass

Answer 3

• Conventional forestry
• Short rotation crops (e.g. poplar, willow)
• High Carbon Percentage : 48,8
• High Hydrogen Percentage: 6,2
• High Oxygen Percentage: 44,2

Question 4

Main molecular components of wood biomass

Answer 4

• Cellulose: absorption of tractive forces
• Hemicellulose: intercellular cement
• Lignin: absorption of compression forces

Question 5

How is crude oil formed?

Answer 5

• Crude oil is formed mainly from marine biomass (algae). Main molecules are easy degradable hydrocarbons (e.g. agarose), proteins and fatty acids
• The sedimented biomass is converted under the
influence of pressure and temperature to crude oil (60 – 120°C) and natural gas (170 – 200°C)
• Actual known oil reservoirs have been built 20 – 350
million years ago

Question 6

What are the main chemical components of crude oil?

Answer 6

4 main groups of hydrocarbons :
• alkanes (paraffines)
• alkenes (olefines)
• cycloalkanes
• arenes
Crude oil is mostly mixed with water (up to 90%), so one of the first processing step is dewatering
Composition of dewatered crude oil:
• Carbon: 85 – 90 %
• Hydrogen: 10 – 14 %
• Sulfur: 0,2 – 3,0 % (max. 7)
• Nitrogen: 0,1 – 0,5 % (max. 2)
• Oxygen: 0,0 – 1,5 %

Question 7

Describe the conversion steps of the biogas process

Answer 7

• Hydrolyses: cracking of macromolecules
• Acidogenesis: fermentation of monomers
• Acetogenesis: formation of methanogenic substances
• Methanogenesis: formation of biogas

Question 8

Show the 2 possible reaction pathways of methanogenesis

Answer 8

CH3COOH = CH4 + CO2

CO2 + 4H2 = CH4 + 4H2O

Question 9

Explain Substrate Inhibition in the context of biogas production

Answer 9

• for an optimal biogas production, the concentration of
organic dry matter (odm) in the fermenter should be
2,0 – 5,0 kg/m³*d
• odm <2,0: fermenter is „hungry“
• odm >5,0: fermenter is overfeeded (acidogeneses => pH ↓)

Question 10

Explain Product Inhibition in the context of biogas production

Answer 10

• with normal methanogenesis, the concentration of
carboxylic acids is < 1.000 mg/l
• with an amount of > 2.000 mg/l, methanogenesis is
inhibited significantly
• an amount of > 10.000 mg/l is absolutely toxic,
methanogenesis stops
• if the fermenter is not sufficiently degassed, the
enclosed methane will inhibit the methanogenesis

Question 11

What are the main components of a bioethanol plant (Schema)?

Answer 11

• Milling/ Chipping
• Hydrolyses
• Fermentation
• Distillation
   • Concentration
   • Drying
   • Pelletizing => Distilled dried grains with solubles 
      DDGS´s
• Rectification
• Dehydration
• Bioethanol

Question 12

Describe the pre-treatment if cellulose containing feedstock is used

Answer 12

• milling/chipping/dispersion
• chemical/enzymatic hydrolysis
• lignin separation
=> mash, C5+C6-sugar

Question 13

Conversion steps of combuston:

Answer 13

• Heating/Drying: Low temperature no air needed
• Pyrolyses: Used to form Coal. λ = 0
• Gasification: High temperature, but not enough air for combustion (Partial Combustion). 0 < λ < 1
• Oxidation: More Air than needed (Full oxidation). λ > 1

Question 14

Describe the conversion steps of combuston: Heatig/Drying

Answer 14

• Drying phase up to 200°C
  
  • Evaporation of the surface water
  • Cracking of the capillar structures (heating rate!)
  • Evaporation of the capillaric water
• Organic mass is mainly unchanged
  
  • No chemical reactions
  • Softening of resins
• Endothermic process
  
  • External heating or
  • Partial combustion of the fuel necessary

Question 15

Describe the conversion steps of combuston:

Pyrolyses

Answer 15

• Phase 1 (200 to 280°C):
• Start of the pyrolytic decomposition
• CO2 and volatile organic acids are escaping
• Endothermic process
• Phase 2 (280°C to 500°C)
• Thermo-cracking of the macro-molecules
• Exothermic prozess under formation of burnable
gases (methane, CO, methanol, hydrogen) and char
• At 400°C starts the building of cristallised graphite
=> transition to endothermic range

Question 16

Describe the conversion steps of combuston:

Gasification

Answer 16

• Gasification reactions (700 - 900°C)
  
  • Cracking of the pyrolysis gases
  • Degradation of the pyrolysis char
  • Formation of the product gas components CO, H2 and CH4
• Gasification reactions are
  
  • Equilibrium reactions
  • Homogeneous and heterogeneous
  • Endothermic and exothermic

Question 17

How is lambda defined?

Answer 17

Air - Fuel ratio
Actual amount of air over the stoichiometric demand.
λ = Air/Air (stoich)

Question 18

Give an outline of the material balance (elements, molecules) of a gasification process.

Answer 18

Feed: C, O, N, H, S, H2O, Ash
Air: O2, N2
Product gas: CO, CO2, CxHy, CH4, H2, H2O, H2S, NH3
Char: Ash, C

Question 19

Describe why straw is difficult to use for biogas production.

Answer 19

• Cellulose (e.g. straw) has to be pre-treated by chemical hydrolyses

Question 20

Describe why straw is difficult to use as a fuel of combustion.

Answer 20

• Effort for exhaust gas treatment depends on the Fuel quality. Straw has a higher Chlorine, Nitrogen and Sulfur content.