Selection of an appropriate biogas storage system makes a significant contribution to the efficiency and safety of a biogas plant. A biogas storage system compensates fluctuations in the production and consumption of biogas as well as temperature-related changes in volume. There are two broad categories of biogas storage systems
: Internal Biogas Storage Tanks are integrated into the anaerobic digester while External Biogas Holders are separated from the digester forming autonomous components of a biogas plant.
Types of Biogas Storage Systems
The simplest and least expensive storage systems for on-site applications and intermediate storage of biogas are low-pressure systems. Floating gas holders on the digester form a low-pressure storage option for biogas systems. These systems typically operate at pressures below 2 psi.
Floating gas holders can be made of steel, fiberglass, or a flexible fabric. A separate tank may be used with a floating gas holder for the storage of the digestate and also storage of the raw biogas. A major advantage of a digester with an integral gas storage component is the reduced capital cost of the system.
The least expensive and most trouble-free gas holder is the flexible inflatable fabric top, as it does not react with the H2S in the biogas and is integral to the digester. These types of covers are often used with plug-flow and complete-mix digesters. Flexible membrane materials commonly used for these gas holders include high-density polyethylene (HDPE), low-density polyethylene (LDPE), linear low density polyethylene (LLDPE), and chlorosulfonated polyethylene covered polyester. Thickness for cover materials typically vary from 0.5 to 2.5 millimeters.
The frequently used low pressure tanks have an overpressure range of 0.05 to 0.5 mbar and are made of special membranes, which must meet a number of safety requirements. The membrane tanks are installed as external gas reservoirs or as gas domes/covers, in top of the digester. External low-pressure reservoirs can be designed in the shape of membrane cushions/ gas balloons. The membrane cushions are placed in buildings for weather protection or equipped with a second membrane.
Points to Consider
If the digester or the post-digester is used for biogas storage, both must be covered with gas tight membrane domes (double membrane reservoirs), fixed on the upper edge of the digester. A supporting frame can be installed in the digester to hold the membrane when it is empty. The membrane expands according to the volume of gas contained. In order to limit the membrane expansion, a special net can be mounted over it
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