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Difference between revisions of "Anaerobic digestion"
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=== Process === | === Process === | ||
There are three basic anaerobic digestion processes, namely psychrophilic, mesophilic, and thermophilic, which take place over different temperature ranges. Psychrophilic digestion is a low temperature (<20°C) process. Mesophilic digestion takes place between 20 and 45°C, which can take a month or two to complete, and thermophilic digestion between 45 and 65°C, which is faster, but its micro-organisms are more sensitive. The majority of the agricultural biogas plants are operated at mesophilic temperatures. Thermophilic temperatures are applied mainly in large-scale centralised biogas plants with co-digestion<ref>{{Cite web|year=2021|title=Anaerobic digestion|e-pub date=2021|date accessed=6/9/2021|url=https://www.eubia.org/cms/wiki-biomass/anaerobic-digestion/}}</ref>. | There are three basic anaerobic digestion processes, namely psychrophilic, mesophilic, and thermophilic, which take place over different temperature ranges. Psychrophilic digestion is a low temperature (<20°C) process. Mesophilic digestion takes place between 20 and 45°C, which can take a month or two to complete, and thermophilic digestion between 45 and 65°C, which is faster, but its micro-organisms are more sensitive. The majority of the agricultural biogas plants are operated at mesophilic temperatures. Thermophilic temperatures are applied mainly in large-scale centralised biogas plants with co-digestion<ref>{{Cite web|year=2021|title=Anaerobic digestion|e-pub date=2021|date accessed=6/9/2021|url=https://www.eubia.org/cms/wiki-biomass/anaerobic-digestion/}}</ref>. The process of anaerobic digestion takes place through four successive stages: hdyrolysis, fermentation, acetogenesis, and methanogenesis.<ref>{{Cite journal|author=Junye Wang|year=2014|title=Decentralized biogas technology of anaerobic digestion and farm ecosystem: opportunities and challenges|journal=Fronties in Energy Research|volume=2|page=|doi=10.3389/fenrg.2014.00010}}</ref> In the hydrolysis step, the feedstock is broken down into soluble substrates (e.g., sugar and amino acids) by enzymes. Fermentation involves the conversion of sugar, amino acids, and fatty acids into ammonia, organic acids, hydrogen (H<sub>2</sub>) and CO<sub>2</sub>. In the acetogenesis step, volatile fatty acids are broken down into acetic acids, CO<sub>2</sub> and H<sub>2</sub>. Finally, methanogenesis step converts acetate, formaldeyde, and H<sub>2</sub> to CH<sub>4</sub> and water<ref>{{Cite journal|author=Jay N. Meegoda, Brian Li, Kush Patel, Lily B. Wang|year=2018|title=A review of the Processes, Parameters, and Optimization of Anaerobic Digestion|journal=International Journal of Environmental Research and Public Health|volume=15|page=|doi=10.3390/ijerph15102224}}</ref>. | ||
[[File:Anaerobic stages.png|thumb|Simplified scheme of pathways in anaerobic digestion (not own work)]] | |||
Usually, the produced biogas must be dried and drained for condense water and biological or chemical cleaned for H<sub>2</sub>S, NH<sub>3</sub> and trace elements. Further upgrading of the biogas to increase the CH<sub>4</sub> content could be realized by membrane separation of CO<sub>2</sub> and pressurising the biogas. | |||
== Product == | == Product == | ||
Anaerobic digestion produces two valuable outputs, namely biogas and digestate. Biogas is composed of methane (CH<sub>4</sub>), which is the primary component of natural gas, at a relatively high percentage (50 to 75%), carbon dioxide (CO<sub>2</sub>), hydrogen sulfide (H<sub>2</sub>S), water vapor, and trace amounts of other gases. The energy in biogas can be used like natural gas to provide heat, generate electricity, and power cooling systems. Biogas can also be purified by removing the inert or low-value constituents (CO<sub>2</sub>, water, H<sub>2</sub>S, etc.) to generate renewable natural gas (RNG). This can be sold and injected into the natural gas distribution system, compressed and used as vehicle fuel, or processed further to generate alternative transportation fuel or other advanced biochemicals and bioproducts. | Anaerobic digestion produces two valuable outputs, namely biogas and digestate. Biogas is composed of methane (CH<sub>4</sub>), which is the primary component of natural gas, at a relatively high percentage (50 to 75%), carbon dioxide (CO<sub>2</sub>), hydrogen sulfide (H<sub>2</sub>S), water vapor, and trace amounts of other gases. The energy in biogas can be used like natural gas to provide heat, generate electricity, and power cooling systems. Biogas can also be purified by removing the inert or low-value constituents (CO<sub>2</sub>, water, H<sub>2</sub>S, etc.) to generate renewable natural gas (RNG). This can be sold and injected into the natural gas distribution system, compressed and used as vehicle fuel, or processed further to generate alternative transportation fuel or other advanced biochemicals and bioproducts. | ||