Difference aerobic and anaerobic fermentation
Aerobic = with oxygen
Anaerobic = without oxygen
Biogas Process Basics - The Anaerobic fermentation
Anaerobic fermented biogas consists of 45-70% CH4 and 25-55% CO2.
Biological substrate used: Slurry, manure and renewable raw material (silage)
Slurry is semi-liquid mixture of animal feaces and urine
Manure is moist/dry mixture of animal feaces with straw, hay or saw dust
Process Steps for Anaerobic Fermentation
Complex, 4 stage process —> microbiologic, chemical and physical influence quantities
First Stage: Hydrolysis
Hydrolytic bacteria + enzymes break down into complex compounds (protein, carbohydrates,fats) to simple organic compounds (amino acids, sugar, fatty acids)
—> From original material to simpel organic components
Used bacteria: Hydrolytic bacteria
Second Stage: Formation of acid / Acidogenesis
Acid forming bacteria convert the simple organic compounds to lower fatty acids (propionic acid, butyric acids, CO2, H2) and small amounts of acetic and lactic acids
Simple organic components (amino acids, fatty acids, sugar)
—> Lower fatty acids (propionic acid, butyric acid)
—> Further products (lacticum acidum, alcohols)
Used bacteria: Fermentative bacteria
Third Stage: Formation of acetic acid / Acetogenesis
Products: Acetic acids and H2 + CO2
Partial pressure of hydrogen: If hydrogen content is too high it is bad for acetic acid bacteria —> must go into close relationship with methane-forming bacteria (4. stage)
Used bacteria: Acetogenetic bacteria
Fourth Stage: Formation of Methane / Methanogenesis
Products are formed from products of acetogenesis (acetic acids, hydrogen, CO2)
Products: Biogas (CH4 + CO2)
All microorganisms have different requirements (temperature, humidity, air, pH and pressure) —> in all stages —> But 4th stage is the most sensitive
Used bacteria: Methanogentic archeas
Decomposition
One-step: All steps of fermentation in one digester
Wet fermentation (dry substrate content 5-12%)
Advantage: High gas quality, heat value until 20 MJ/m3
Energy Balance and Efficiency
Carbon Footprint
Environmental Conditions
Humidity - Certain levels of humidity is always needed
Oxygen - Insertion of air cannot be fully avoided for anaerobic fermentation
Temperature
For stable fermentation optimum temperature for mesophil bacteria is 30-45C
For fast fermentation optimum temperature for thermophil bacteria is 45-55C
pH
pH level for hydrolyzed and acid-forming is 5-6.3 —> Can tolerate higher pressure
pH level for acetic acid-forming and methanogens is 6.5-8 —> more sensitive
optimum pH-value for plant is 7
Parameters of Operation
—> Factors for optimization / construction
Digester Loading- rate at which substrate is fed into the anaerobic digester
Degradation degree- rate to which substrate in digester is broken down by microorganisms
Hydraulic Retention time - average time the liquid portion of the substrate remains in the digester
Methane production
Digester Loading
Formular for calculating dry matter content.
Digester Loading: Biogas Loading Rate
BR is amount of organic dry matter per m³ which is added to the digester daily.
Hydraulic Retention Time
—> The higher the digester loading the lower the retention time
—> Gas yield depends on the retention time
Minimum time: low time —> bacteria become less —> stop of the process
Maximum time: Regarding to economic aspects
Gas Quality
Useful Terms
Floating layers
Inhibitors
Intermediates
Retardants
—> Too dry or too light material rises and forms coherent layers at the top making fermentation process to fall down. Can be prevented by stirring.
—> Negatively affect digestion process.
—> Compounds that form as byproducts during the multi-step process of anaerobic digestion.
—>substances that inhibit the anaerobic digestion process, reducing efficiency and biogas yield.
Last changed4 months ago