Temperature effect on biogas production
Biogas production from organic substrates is strongly affected by the temperature where anaerobic digestion takes place.
The process of organic material anaerobic digestion takes place in three main temperature ranges: from 10-25°C (psychrophilic conditions), from 30-37°C( mesophilic conditions) and from 48-55°C ( thermophilic conditions). In practice, however, when the target is the most efficient production of biogas and its use for electric and thermal energy production from the cogeneration system only the mesophilic and thermophilic anaerobic digestion is applied. Generally at very low or high temperatures (e.g. temperatures below -10 ° C or above 90 ° C) microorganisms destroyed completely.
The main reason for rejecting the psychrophilic process is that at such low temperatures the rate of organic matter conversion into biogas is minimized, since the activity of microorganisms is limited. The consequence is to require very large residence times in the digesters and hence very large volumes of reactors and tanks. Even so, biogas typical components which result lead to degraded fuel energy content. Most applications of psychrophilic processes are usually found in tropical areas thereby maintaining the cost of construction of biogas plants quite low, while there are no requirements for producing high quality fuel as it is mainly used for domestic purposes.
The majority of methanogens (the microorganisms that form methane from organic matter) belong to the mesophilic. They grow quickly in this temperature range and exhibit high degrees of conversion. In practice, this has direct implications in the design of biogas plants as they are the most stable operating plants. The stability and growth conditions in the digester at mesophilic conditions make the process more balanced, more resistant to chemicals that inhibit digestion (e.g. ammonia) and capable of treating efficiently a great variety of different types of biomass and waste; even the most difficult treated like the slaughterhouse waste or the animal by-products.
A smaller proportion of methanogenic organisms are thermophilic, meaning that are attached perfectly to higher temperatures. Generally, at these temperatures all bacteria consume the organic substrate with higher rates and grow faster. Because of this, the digesters operated at thermophilic conditions may be constructed in smaller dimensions (which means lower manufacturing costs) while maintaining very high levels of biogas. Equally positive feature of thermophilic anaerobic digestion is the possibility of killing a higher proportion of pathogenic bacteria present in organic wastes (e.g. manure, slaughter waste).
Despite the advantages of the thermophilic process, mesophilic anaerobic digestion continues to dominate as an option in most biogas plants. This choice can be justified to the more difficult control and optimization of the thermophilic process. The thermophilic methanogenic bacteria are extremely sensitive to changes in anaerobic digestion to such an extent that even a small change of the operating parameters can impact negatively on their development. For example, a change in temperature greater than 1-2 ° C has a significant reduction in the amount of produced biogas. Moreover, the variety of materials that can be processed in anaerobic thermophilic conditions is lower than the mesophilic, mainly because of the chemical composition and the stronger influence of some digestion inhibitors in the process.
In their effort to combine the advantages of each process, many modern biogas plants choose to implement anaerobic digestion in two stages: a mesophilic one and a thermophilic. Although the cost of investment may increase, the plants are benefited from the advantages of each type of digestion, thus optimizing the quality and the amount of biogas.
The biomass feedstock selection in a biogas plant is crucial for the anaerobic digestion process. Animal manure and energy crops is the most popular feedstock in farm-scale biogas plants. more