Digester Heating

Overview

To achieve the optimum biogas yield, the anaerobic digestion needs constant environmental conditions, preferably close to the process optimum. The digester temperature is of prime importance. In temperate areas, a heating system and an insulation of the digester is necessary. Hence, the needed temperature for digestion can be reached and a loss of energy by transmission is compensated.

There are a number of different ways to get the required amount of thermal energy into the substrate.

In principle, one can differentiate between:

• direct heating in the form of steam or hot water, and
• indirect heating via 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

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.

1. Floor heating systems have not served well in the past, because the accumulation of sediment gradually hampers the transfer of heat.
2. 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.
3. 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. 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.
4. Ex-vessel heat exchangers offer the advantage of easy access for cleaning and maintenance.

While in Northern countries, often a substantial amount of the produced biogas is consumed to provide process energy, in countries with higher temperatures and longer sunshine hours, solar-heated water can be a cost-effective solution for heating. Exposing the site of the biogas plant to sunshine, e.g. by avoiding tree shade, is the simplest method of heating.

Costs

Because of the high costs for material and installation of a heating system, a low-cost biogas plant, as needed in developing countries, can only be build without heating. To boost the biogas yield for those plants, the building of a bigger digester to increase the retention time would be cheaper. A bigger digester reduces the required maintenance, while a heating system, increases maintenance requirements. A bigger digester serves also as a buffer for sediments, pH-variations and gas storage. For example, a fixed dome plant sized 50% bigger, is only 10% more expensive.

Photo: Heating system for a biodigester under construction (Germany)

Influencing Factors

The mean surrounding temperature and it's seasonal variations are very important. Biogas plants without heating system work, therefore, only in warmer regions for the whole year. In regions with extreme temperature variations, for instance in Turkey (hot summer, cold winter), the biogas plant should be built under the stable. Hence the biogas yield would be lower in summer, but constant over the year and at the end higher. Before implementation, at least an approximated average temperature profile and expected extremes over the year should be available for the site.

A biogas plant with heating system and co-generation can be operated with process energy. Nevertheless the dimensioning of such a heating system is difficult, as the substrate, which has to be heated up, is not homogenous.

A guiding figure for a digester with a hydraulic retention time of 20 days is 270 W/m³ digester volume. The increasing of the hydraulic retention time makes it possible to reduce the heating power per volume. With a hydraulic retention time of 40 days the digester needs only 150 W/m³.

Following figures are for heating systems with a heating water temperature difference of 20 K:

hydr. retention time	40 days	30 days	20 days
temperature difference	20 K	20 K	20 K
heating power	150 kW/m3	210 kW/m3	270 kW/m3

A heating system located in the digester produces a thermal circulation, which is, especially for non-agitated digesters, very important.

An indirect energy transfer by heat exchanger is most common. Exceptions are steam injection, liquefying of solid manure with heated water and the heating by pre-aeration.

Internal and External Heating Systems

External heating systems have a forced flow on both sides. Due to the turbulent flow patterns of both media, a very good heat transportation can be reached. Therefore, the surface of the heat exchanger can be comparatively small. Nevertheless those systems cannot be recommended for non-agitated digesters.

The proper dimensioning of an internal heating system seems to be more difficult because of the different currents due to pumping, agitation, thermo-convection and the inflow of bio-mass.

Under-floor heating systems have been very popular, as they have no disturbing parts in the digester itself. Due to sedimentation and the resulting worsening of heat transportation into the digester, under-floor heating is no longer recommended. With the growth of digester volumes and the need of bigger heating systems, it is also more difficult to build under-floor heating big enough to provide the necessary heat.

Heating coils installed at the inner wall of the digester are a rather new practice. Heating coils made out of steel are much more expensive than heating coils out of plastic material (PE). Materials developed during the last years make such a system more stable while not increasing the costs of the heating system.

Another option is to construct two digesters connected in series, the first heated, the second unheated. The first digester can be used as sedimentation tank , in which the substrate is heated up. The second digester is well isolated to reduce loss of heat.

Heat Transport in Biogas Plants

Combined Heat and Power (CHP)

Combined heat and power (CHP), or cogeneration, refers to the simultaneous generation of both heat and electricity. Depending on the circumstances, a distinction can be drawn between power-led and heat-led CHP plants. The heat-led type should normally be
chosen, because of its higher efficiency. In almost all cases this means using small-scale packaged CHP units with internal combustion engines coupled to a generator. The engines run at a constant speed so that the directly coupled generator can provide electrical energy that is compatible with system frequency. Looking into the future, for driving the generator it will also be possible to use gas microturbines, Stirling engines or fuel cells as alternatives to the conventional pilot ignition gas engines and gas spark ignition engines.

►Presentation on CHP engines.

Flare Systems

In case the storage tanks are unable to take more biogas and/or the gas cannot be used on account of maintenance work or extremely poor quality, the excess has to be disposed of in a safe manner. In Germany, the regulations relating to the operating permit vary from state to state, but installation of an alternative to the CHP unit as ultimate sink is required if the gas flow rate is 20 m3/h or higher. This can take the form of a second cogeneration unit (for example two small CHP units instead of one large one). A margin of safety can be established by installing an emergency flare, as a means of ensuring that the gas can be disposed of in an adequate way. In most cases the authorities stipulate that a provision of this nature be made.

High Temperature Flares have to be obligatory not only for CDM projects to destroy surplus biogas which cant be utilised due to technical problems or overproduction. Investment costs about 1% of the total investment.

Further Information

• Biogas Portal on energypedia