Biogas is basically a mixture of methane and carbon di-oxide gases, generally produced from cattle dung, agriculture manure, MSW etc by anaerobic digestion process. This technology has many benefits in view of environment, agriculture and sustainability. Biogas plants have been extensively used in India. There are certain parameters that affect production of biogas plant. Temperature is one of them. Although anaerobic digestion can take place under psychrophilic (15-25°C), mesophilic (35-40°C) and thermophilic (50-60°C) conditions. Temperatures of 35-37°C are typically recommended for best methane production. As the temperature varies, percentage of methane decreases. However, digesters require significant amount of heat energy to maintain temperatures at these levels. There are various technologies available to increase the temperature of digester. Use of solar energy is one of the cheapest ways.
Description of Solar Heating System for Biogas Production
Solar air/water collectors combined with a heat exchanger can be used for heating digester and increase temperature to an optimum value in a steady condition. In order to keep the system run in steady condition, the control unit can be adopted. If there is not enough solar thermal energy input to the digester (e.g. cloudy season), then the auxiliary electrical heating system starts to supply the heat to the digester. The digester is generally positioned under the ground. There are basically two methods that are most efficient to heat the biogas digester.
Schematic view of Biogas Plant
Use of Solar Water Heater
In this design, the digester is kept underground and heated via heat exchanger using the hot water from the solar water heaters so as to maintain the desired temperature for better production of biogas. Efficient system if enough sunlight is available as it serves to increase the digester’s reaction temperature and/or reduce reactor’s volume.
For such applications double tube heat exchangers can be used. It is an industrial double tube heat exchanger, comprising a tube within a tube. The inner tube is corrugated for increased heat transfer and reduced fouling without the risk of obstruction or blockages associated with spiral heat exchanger systems, thus, ensuring continuous operation in such a harsh environment. The product flows through the inner tube and the service fluid through the annulus between the inner and outer tube. Because of its geometry, the double tube heat exchanger is a true counter-current heat exchanger. An expansion joint (bellow) is fitted in the shell to allow for differential expansion of the inner and outer tube during operation. Multiple units can be interconnected and have the options of frame mounting, insulation and cladding in stainless steel.
Schematic layout of utilization of solar water heater with heat exchanger in biogas digester
Solar Canopy
Polythene/plastic sheet is used as a canopy for tapping solar energy. This method is used to enhance the biogas production for the low temperature in winter. This method is the most effective methodology in plan area. A black plastic hut is made over the dome of a biogas digester so that solar radiation is absorbed by the hut and consequently rising the temperature of the digester. The contents of the digester must be heated to maintain the ideal temperature for the bacteria to work. In addition, pre-heating the feedstock prior to putting it in the digester can reduce the amount of heat needed in the digester itself and improve the overall efficiency of the digestion process.
HRS make double tube heat exchanger
Conclusion
There are many different ways to maximize both the efficiency of biogas plants, and the usefulness and effectiveness of digester. With the right advice and by choosing the correct technology, it is possible to enhance the production of biogas which is not only increase plant efficient, but also improve the overall environmental profile of biogas plants and maximize its benefits in terms of mitigating climate change.
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