Biogas plants - Electric and thermal energy production from the cogeneration system

Energy production from biogas plantThe last part of a typical biogas plant is consisted of the Combined Heat and Power (CHP) system.

During that phase the produced biogas, after it has been firstly cleaned from all the undesirable compounds, is led to an internal combustion gas engine, where it is burned.

The engine is properly connected with an electric power generator from where clean, renewable energy is produced.

The heat recovery from the exhaust gases and the cooling system results into the production of thermal energy, too, either in the form of steam or as hot water. In the majority of biogas plants, the CHP system is installed in properly constructed containers, nearby to the anaerobic digesters.

The main characteristic of CHP systems is their ability to produce simultaneously both electric and thermal energy. Thereby, the total efficiency of a CHP system is much higher compared to the conventional electric power generator. As a matter of fact, it can even reach values of 85-90% efficiency when the latter appear rarely values above 40%.

More specifically, depending on the system’s capacity, the electric output of a typical CHP engine may vary between 33-40%. Generally speaking, by increasing the capacity the efficiency is increased too. For biogas plants with a relatively small capacity (<200 kWe), the combination of biogas with fuel oil or biodiesel (in a 10% blend) is a quiet common practice in order to increase CHP total efficiency.

The generated electric energy can be sold to the national grid with a feed-in-value which is predefined by law. On the other hand, the commercial exploitation of thermal energy is a more complex procedure.

Since anaerobic digestion is taking place in moderate temperatures (either 35-38 or 53-56 °C) the use of a portion from the heat produced in order to keep the digester under constant temperature conditions is necessary. At the same time, if the plant processes materials to which is obligatory to apply thermal pre-treatment (e.g. animal by-products), then the demands for heat can be covered with self consumption from the CHP system. The efficient digestate management is among the factors that affect the whole investment profitability. A method which is applied sometimes consists of the evaporation of the excess moisture content of digestate so as to be easier handled.

From the abovementioned facts, it is assumed that depending on the type of the biogas plant, its heat requirements can be significant. Thus the exploitation of thermal energy produced from the CHP system (self-consumption) assists to the considerable decrease of biogas plant’s operational costs.

Apart from self-consumption, the potential of selling the produced thermal energy to nearby residencies or industries through a district heat network is a great challenge which may multiply the benefits both for the investment economics and the environment.

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