Micro-scale digesters in practice
Micro-scale digesters produce biogas using only own (waste) biomass resources from farms, where livestock manure is the main substrate. Biogas is produced as a result of a fermentation process – the organic material in the substrates is reduced and converted to biogas by micro-organisms.
The biogas is mostly utilised for energy generation – heat (produced in boilers) or heat and power (generated in combined heat and power (CHP) installations). There are several different ways of biogas usage. One of them is upgrading to biomethane, which can be used as transport fuel or fed into the natural gas grid.
Digested manure is used as a fertilizer spread on farmland. It has many advantages over raw manure (i.a. unpleasant odours are reduced). Micro-scale digesters are renewable energy sources, which contributes to reduction of greenhouse gas emissions and decrease of fossil fuels use.
Figure: Estimated number of operating agricultural micro-scale digesters identified in selected countries according to a survey by BioEnergy Farm 2 project partners.
How does MSD work?
Figure: Manure digesting – scheme. Source: NAPE.
The chain utilizing of the manure starts from the livestock house. The manure is temporarily stored in a reception tank, and then fed to the digester. During the fermentation process in the digester biogas is produced. When the retention time in the digester (post digester) has elapsed, the digestate is pumped into a storage tank, and then utilized on the farmland as fertilizer.
Biogas can be utilized in a number of ways but generally it is used to generate heat and power in micro combined heat and power generators (CHP) or only for producing heat in a boiler. The produced electricity and heat is used by farmers for household and agricultural purposes. Because the production is small, sufficient heat demand should be found on the farm and in the neighbourhood. Eventual electricity surplus can be fed to the grid.
If there is no heat demand in the near area of CHP installation, the biogas, instead of heat and electricity production, could be upgraded to transport fuel or grid gas standards. However, it requires additional support in legal frameworks (a.i. regulations and injection criteria).
To summarize, the most feasible solution of biogas utilization depends on specific local conditions, but usually biogas is used for producing heat or combined heat and power…
How does a MSD look like?
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Micro-scale digestion as an important part of the EU energy strategy