Biogas Collection, Storage & Safety Considerations
Biogas Collection: Collecting the Biogas
As mentioned in the previous article, digester gas is typically comprised of 60-70% methane, with the balance consisting primarily of carbon dioxide together with smaller amounts of hydrogen sulfide and other trace gases. To prevent the biogas from being contaminated by atmospheric oxygen water levels need to be maintained so that at least three inches of water covers the liquid waste in the biodigester tank as well as the gas collection system.
This can be achieved by using either one of the following two gas collecting methods:
- Using a floating cover that floats on the biogas compressed between the liquid waste and the cover. This ensures a suitable pressure for distributing the gas to the storage facility.
- Using a pressure regulator that releases the biogas from the biodigester once a specified pressure has been attained.
Any piping used to collect and/or distribute the gas should be designed so that they either run at an angle back down into the digester, or are fitted with condensate traps that prevents any water vapor produced as the gas cools down from forming condensate that can block the gas collection/distribution lines. A inline direct mass flow meter should be installed onto the gas collection line to enable monitoring of digester operations; gas production levels that are high and stable indicate that the system is operating well. The verification process for the gas production needs to be verified by using a BioGas Analyzer.
Storing the Biogas
Gas storage tanks of varying volumes should be installed for changes in gas production (as well as gas consumption) rates while still maintaining a stable pressure. The most viable method of reducing the costs associated with gas storage is to utilize the gas at the same rate that it is produced.
- Floating Digester Cover: Using a floating pontoon cover in the digester, as mentioned above, can serve as a gas collection and gas storage mechanism.
- Gas Storage Tank: A high-pressure storage vessel can also be used, but as this is quite costly for small-scale farm use, a medium-pressure storage vessel (<100 psi) may be a more viable option. A biogas storage tank is typically a spherical or cylindrical shaped unit constructed from welded steel. Ensure that someone who is experienced with the design and construction of pressurized vessels is consulted due to the risk of leakage and/or explosion associated with pressurized gas.
Reducing Odor & Corrosion
If the waste in the digester contains a high level of sulfates, hydrogen sulfide â€” which smells like rotten eggs â€” can be produced. However, if the digester is operating properly, odors should be minimal as both methane and CO2 are odorless gases, and are stored within an airtight storage vessel before being burnt, eliminating any problems associated with odor.
Hydrogen sulfide is also highly corrosive, and thus corrosion can pose a real problem if measures are not taken to minimize it. To this end, hydrogen sulfide is best removed using specialized filters (filters filled with lead filings; a woodchip and iron oxide mix; or Sorb beads are typically used to remove hydrogen sulfide).
Methane is an extremely volatile gas, and when it mixes with air (at just 6-15% methane concentrations) there is a risk of explosion. Biogas is heavier than air, and as a results sinks, displacing oxygen near the ground. When hydrogen sulfide is present, this gas not only presents an explosive threat, but can also pose a poison hazard that could prove deadly in confined spaces. To ensure proper safety, if the biodigester is housed indoors make sure that there is adequate ventilation, and that pressure gauges and/or pressure relief valves are vented to the outside. It is important to have a Methane Gas Detector capable of measuring the values.
Always take care when compressing or storing biogas, ensuring that suitable equipment and specialized storage tanks are used when biogas is stored under high pressure.
Featured Image by Vortexrealm at en.wikipedia, [CC BY 3.0]