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Substrate heating systems

Many small-scale biogas plants in Kyrgyzstan have been built without insulation and heating system. Absence of heating system allows biogas plant to operate only in psychophilic temperature range and to generate less biogas and fertilizer, than under mesophilic and thermophilic temperatures.

For allowing for higher biogas and fertilizer production and for better decontamination of substrate two methods of heating an be used: direct heating with steam or mixing substrate with hot water indirect heating through heat exchanger whereby the heating medium, usually hot water, imparts heat while not mixing with the substrate.

Direct heating
Direct heating with steam has the serious disadvantage of requiring an elaborate steam generating system (including desalination and ion exchange as water pretreatment) and can also cause local overheating. The high cost is only justifiable for large-scale sewage treatment facilities. The injection of hot water raises the water content of the slurry and should only be practiced if such dilution is necessary.

Indirect heating

Figure 26. Equipment for indirect heating of the substrate

Indirect heating is accomplished with heat exchangers located either inside or outside of the digester, depending on the shape of the vessel, the type of substrate used, and the nature of the operating mode.

Floor heating systems have not served well in the past, because the accumulation of sediment gradually hampers the transfer of heat. In-vessel heat exchangers are a good solution from the standpoint of heat transfer as long as they are able to withstand the mechanical stress caused by the mixer, circulating pump, etc.

The larger the heat-exchange surface, the more uniformly heat distribution can be effected which is better for the biological process (Figure 26). On-vessel heat exchangers with the heat conductors located in or on the vessel walls are inferior to in-vessel-exchangers as far as heat-transfer efficiency is concerned, since too much heat is lost to the surroundings.

Figure 27. Water boiler of heating system in Petrovka village.
Photo: Vedenev А.G., PF «Fluid»

On the other hand, practically the entire wall area of the vessel can be used as a heat-transfer surface, and there are
no obstructions in the vessel to impede the flow of slurry. Ex-vessel heat exchangers offer the advantage of easy access for cleaning and maintenance.

In-vessel and ex-vessel heating systems
To achieve the optimum biogas yield, the anaerobic digestion needs constant environmental
conditions, preferably close to the process optimum. In Kyrgyz Republic insulation and heating system are necessary for reaching optimum digestion temperature and preventing energy losses. For heating the digester to mesophilic temperature with electricity on average it is necessary to spend 330 W for 1 m3 of digester volume.
The most common heating system is an ex-vessel system with water boiler that works on biogas, electricity of solid fuel.

As heating elements coil-shaped heat exchangers, radiator sections of pipes welded parallel to each other, where water heated up to 60ºС is used as a heating medium. Higher temperature increases the risk of solid particles attaching themselves to the surface of the heat exchanger.

Mounting the heating system
When mounting the heating system it is very important to create conditions favorable for natural movement of substrate inside the digester. For this purpose hot water is fed into the higher point of heating system and returns to lower point cooled.

Piping system for heating has to be equipped with vents for letting the air out of the top points and the heating system has to include overflow tank for compensation of water volume variations. Thermometer has to be installed inside digester to control the inside temperature.