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The changing reputation of digester gas
Digester gas is actually a natural product that occurs in stagnant water and swamps or in agriculture, e.g., cattle farming. In connection with modern waste management, however, this foul-smelling gas has transformed into an important alternative energy source and has thus become socially acceptable. Digester gas is formed from the breakdown of organic, i.e., carbon-containing, substances by bacteria under the exclusion of air through anaerobic fermentation. The carbon reacts to form combustible methane, which makes up to 30-60% of the digester gas and whose energy content of appr. 10 kWh/m3 makes it an attractive energy source. The other components are carbon dioxide (CO2), hydrogen sulfide (H2S), water vapor, and residual gases.
Digester gas from waste treatment and sewage plants
The use of digester gas had a powerful backer in the European Landfill Directive (1999/31/EC), which was adopted in Brussels in 1999 and prohibits disposal of waste in landfills in member countries, with only a few exceptions, and thus makes new waste treatment processes imperative. A transition period until July 2003 was allowed for implementation in national legislation. Landfills had to then be closed or meet new stringent requirements by July 2009. Germany met the deadline with its landfill ordinance and 30th Federal Ordinance for Pollution Control. Among other things, these contain the framework conditions for approval of Mechanical/biological treatment (MBT) plants as an alternative to landfill disposal. A second source of digester gas is digesters in sewage treatment plants, but energy recovery from this gas source is not yet consistently carried out.
The waste to biogas process
After delivery of the household and industrial waste and removal of extraneous materials and non-permitted wastes, the remaining waste material passes through several process steps. In wet treatment, the partial stream is dissolved in water to form a pumpable dispersion, where from heavy particles such as glass, sand, or plastics can be removed. The following biological treatment includes hydrolysis and fermentation steps, whereby most of the organic components are converted to methane, carbon dioxide, water, and bacteria mass. The energy in the resulting biogas is- after desulfurization of the gas - utilized as combustion gas to generate heat and electricity.
Reliable H2S monitoring
Process control is an important issue in plant operation, whereby the analysis of the biogas composition at multiple points of the plant is crucial for the quality and subsequent use of the gas. The biogas composition is subject to strong fluctuations due to the inhomogeneous and seasonally variable sort of the delivered waste. Because of its harmful properties (high toxicity and strong corrosiveness), the concentration of hydrogen sulfide (H2S) in the digester gas and biogas (up to 10 000 ppm at sampling point 1 and only few ppm at sampling point 4) is a crucial factor in the usability of the biogas. The Multigas-Analyzer INCA is an extremely flexible gas analyzer and, thanks to its patented sensing/flushing technology, is able to determine H2S reliably over a widespread concentration range.