Biogas production occurs spontaneously in nature whenever matter decomposes without the aid of oxygen: biogas (or biological gas) is the mixture of gas produced through anaerobic fermentation (that is, without oxygen) of organic matter.
Modern biogas plants function like the digestive system of cattle, in which organic matter – in the absence of oxygen – is converted into biogas, consisting mainly of methane and carbon dioxide. The gas produced through this process is used to power co-generation motors which produce electrical and thermal energy.
Biogas plants are powered with agricultural and agri-food by-products, with livestock manure and energy crops (maize, triticale, sorghum, etc.). The end result of the fermentation process, besides biogas, is the so-called ‘digestate’ (or ‘biodigestate’), a material with liquid and solid fractions that does not generate foul odours and possesses a high agronomic value; moreover, it returns to the fields as fertiliser, partially replacing synthetic chemical fertilisers.
Biogas, therefore, offers an opportunity for producing energy locally and in a sustainable manner, thereby improving the environmental impact of agricultural activities and livestock farming. Thanks to biogas, a series of by-products coming from these important sectors of the national economy can be elevated to the status of ‘resources’. With the resolution of the European Parliament of 12 March 2008, the European Union recognised that ‘biogas is a vital energy source that contributes to sustainable economic, agricultural and rural development and environmental protection’, while stressing ‘the contribution that biogas can make to reducing the EU’s energy dependence’, and, therefore, encouraging ‘both the EU and Member States to exploit the huge potential of biogas by creating a favourable environment’.
Moreover, it must be highlighted that unlike other renewable sources, biogas is programmable, flexible and storable: programmable, as biogas can be produced 24 hours a day, 365 days a year; flexible, as it can be converted into electrical energy, thermal energy, gas, petrol and fuel; storable, as it can be used in the form of biomethane for the above-mentioned purposes which can be deferred, so as to prevent or limit production peaks, which could cause problems for the various distribution networks (electricity, gas, etc.).
A typical medium-size biogas plant (1.0 MWe) usually consists of one or more loading pre-tanks, one or more digesters, one or more final tanks and a co-generator. The biomasses and livestock manure are introduced daily into the process through the loading pre-tanks in pre-defined mixes and quantities (the so-called plant ‘diet’). These materials are then conveyed to the digesters, namely hermetic tanks in which biological processes lead to the production of biogas without the aid of oxygen. The biogas is purified and sent to the co-generator as ‘fuel’ for the endothermic motor, to which an alternator is connected for producing electricity. A small amount of the heat produced by the co-generator is used to keep the temperature of the digesters constant, while the remaining amount is reserved for other civil or industrial uses. After roughly 30-45 days, the materials – having exhausted their methane-generating potential – are transferred from the digesters to the final tanks. Following a further stabilisation period, the material is separated into a liquid fraction and a solid fraction. Both fractions are reused as organic fertilisers in the fields from which the biomasses used in the plant were originally taken.