F6 Biogas

Question 1

What four substrates have big biogas potential in Sweden?

Answer 1

Crop residues
Manure
Industrial waste
Food waste

Question 2

Definition of biogas process/anaerobic digestion

Answer 2

It is i biological engineering process in which complex organic matters are converted to methane (CH₄) and and carbon dioxide (CO₂) by a mixed population of microbes in the absence of oxygen.

Question 3

Anaerobic environments in nature with low oxygen levels

Answer 3

Sea sediments (sediment på havsbotten)
Wet lands (träskmarker)
Ruminating animals (idisslande djur)
Rice fields

Question 4

Pre-requisites (necessities) for biogas production

Answer 4

Question 5

The flow scheme of anaerobic methane production

Answer 5

Question 6

Hydrolysis for carbohydrates, protein and fat (speed? and product?)

Answer 6

Question 7

Environmental factors for anaerobic methane production

Answer 7

• Nutrient requirements: Macro nutrients (C, O, H, N, S, K, Na…)
  & Micro nutrients (Fe, Zn, Ni, Cu…)
• Toxic compounds (oxygen, heavy metals, organic pollutants, detergents)
• Water activity (>65% water → 35% TS = total solids)
• pH (neutral ca 7-8)
• Redoxpotential (ca -300mV)
• Temperature
  
  • Psychrophilic < 20°C
  • Mesophilic ca 35-40°C
  • Thermophilic ca 55-60°C

Question 8

Process temperatures in biogas digesters

Answer 8

Question 9

Advantages of Thermophilic (60°C) vs Mesophilic (39°C)

Answer 9

Question 10

Different solid concentrations (TS)

Answer 10

Question 11

Digestion by “BATCH” method

How is the biogas flow?
What is the usual feedstock (what process)?

Answer 11

Question 12

CONTINIOUS digester technology

How is the biogas flow?
What is the usual feedstock (what process)?

Answer 12

Question 13

What are the basic substrate characteristics?

Answer 13

Question 14

What does HRT stand for?

Answer 14

It stands for Hydraulic Retention Time - theoretical average residence time of a liquid volume in a CSTR (d) (continuous stirred tank reactor)

HRT = V/Q_o

V = Digester active volume (m³_r)
Qo = volume flow out of the digester (m³/d)
Qi ≈ Qo (for wer fermentation)

Question 15

What does digester active volume mean?

Answer 15

The active volume, is for ex the sludge volume - normally 80-85% of the total volume.

Question 16

How long is the HRT (hydraulic retention time)?

Answer 16

ca 15-20 days depending on the substrate characteristics and digester temperature.

20-50 days is normal for mesophilic processes

Question 17

What does OLR stand for? And what is the mathematical formula?

Answer 17

Question 18

What is the biogas yield (specific biogas production)?

Answer 18

Nm³ biogas/kg VS (volatile solids)

or

Nm³ biogas/kg wet weight

Expresses how well the substrate is utilised

Question 19

What is the volumetric biogas production?

Answer 19

m³ biogas /m³ digester volume/d

Expresses how well the digester volume is utilised

Question 20

Which of carbohydrates, protein and lipids have the highest yield?

Answer 20

Highest → lowest:

Lipids → proteins → carbohydrates

Question 21

What are the main steps at a biogas plant? And what do they conclude?

Answer 21

Question 22

Definitions:

Biogas cleaning?
Biogas upgrading?
Biomethane?

Answer 22

Question 23

Why do we need to upgrade the biogas? And what techniques are there?

Answer 23

Biogas doesn’t meet the requirements of methane volume % → need to upgrade the biogas (remove CO₂).

• Absorption (selective affinity of CO2 solving in a liquid media)
  
  • Water scrubber
  • Chemical scrubber (Amine scrubber)
• Adsorption (selective affinity of CO2 on a media surface)
  
  • Pressure Swing Adsorption (PSA)
• Gas permeation
  
  • Membranes
• Emerging techniques
  
  • Cryogenic distillation (different boiling point)

Question 24

What are the principles of absorption (water scrubbing)?

Answer 24

Question 25

Liquified biomethane/biogas (LBG) +/-

Answer 25

Question 26

Compressed biomethane/biogas (CBG) +/-

Answer 26