8. Biogas production
Learn about the biogas production process in this book.
8.1 Substrates
Many organic materials can be used as substrates for biogas production. The decisive factor for the gas yield, i.e. the biogas or methane yield of a substrate, is the content of water, proteins, fats and carbohydrates as well as their digestibility for microorganisms.
Around 47% of the total substrate mass used today are plants such as maize, grass or grain. Animal excrement makes up around 48% of the substrate mass. Waste (3 %) and residual materials (2 %) are also used to a lesser extent. Cattle manure is particularly suitable for starting the fermentation process because it contains a high level of methane bacteria. Chicken manure produces high gas yields, but its high lime and sand content can lead to deposits in the fermenter, which is why it must be pre-treated.
A total area of 1.57 million hectares was required for the cultivation of the plants used in 2021 (FNR 2022). The figure below shows the share of the various crops in the total area under cultivation.
Characterization of substrates
Pararmeter
Important parameters for determining substrate suitability and the required system design are briefly explained below:
Dry matter (DM or Total solids -TS):
Dry matter = total mass minus the water contained
Dry matter content varies greatly with the substrate
Dry matter content between 25 and 35 % ideal
Organic dry matter (oDM or Volatil solids -VS):
only organic substances can be processed by the microorganisms inorganic substances (e.g. minerals) do not contribute to the methane yield, but can still have a negative impact on the decomposition process due to a lack or excess of them
pH value:
The substrate must be neither too acidic nor too alkaline, as otherwise the composition and interaction of the microorganisms will change pH values of 6 to 8 are ideal, because acid is initially produced during methane formation, but the substrate must not become over-acidic
Biochemical oxygen demand (BOD):
Indicator for the content of organic material available for microorganisms, provides an indication of whether aeration is necessary
Chemical oxygen demand (COD):
represents the organic carbon content, an indicator of the theoretical maximum amount of methane that can be produced from a substrate
Total nitrogen content:
Sum of organic nitrogen and ammonium in the sample
measured using the Kjeldahl method (analysis method named after a Danish chemist)
The correct ratio of carbon (C) and nitrogen (N) is very important (C:N ratio). If there is too much carbon in relation to nitrogen, the carbon cannot be fully converted and the methane yield falls short of the maximum possible yield. Too much nitrogen can in turn lead to the formation of ammonia, which has a toxic effect on some microorganisms and can also cause corrosive damage to the system. A ratio of 30:1 to 10:1 of carbon to nitrogen is considered optimal. High protein or nitrogen concentrations can lead to increased formation of ammonia and hydrogen sulphide, which can cause corrosive damage to the system.
A comprehensive list of the properties of various biogas substrates can be found here.
If you want to find out more about the methods for assessing the suitability of substrates, you can read a guide published by the German Biomass Research Center.Impurities
There are a number of other substances that can inhibit the development of microorganisms. These include disinfectants, antibiotics, zinc, copper and surfactants.