Biogas Production

The biogas plant at Groß Kreutz is used for research and demonstration purposes as well as for informing farmers about the use of renewable energies and, in particular, about the use of biogas. Through partners, such as the Leibniz Institute for Agricultural Engineering and Bioeconomy, for example, a variety of studies on the efficiency of the use of different coferments take place.

1. The structural plant

The construction of the biogas plant "VERA 630 - 40" was started in 2002. Construction delays as well as delivery problems with the equipment company delayed the commissioning on schedule. After completion of the plant with the bulk material reception technology and the necessary control, regulation and measurement technology, it was possible to start feeding into the energy network from 01.10.2003. The complete final acceptance of the biogas plant took place in November 2004. In 2006 the biogas plant was extended from 40 to 80 KW.

The biogas plant consists of:

- a rectangular fermenter made of reinforced concrete with 630 m³ usable volume, which is thermally insulated from the outside and divided by a middle wall in longitudinal direction. On the inner walls there are circumferential pipe heating segments for continuous temperature control. The slurry is fed into the fermenter through a ring channel between the outer and middle walls, supported by two vertical agitators.

- a gas storage tank, designed as a gas-tight and biogas-resistant foil storage tank with a supporting air protection roof. It is mounted directly above the digester.

- the machine room for the combined heat and power unit (CHP), located at the side of the fermenter.

- the machine room for the control of the plant. The feed and discharge lines with the necessary pumping and homogenizing equipment are installed here.

- bulk material storage area with moving floor.

- a gas flare for burning biogas in case of failure of the CHP.

2. Process engineering

The biogas plant operates according to the flow-through process. The feeding of the plant with liquid manure is done by a self-priming rotary lobe pump. When co-ferments are added, they are conveyed to the homogenizer via the moving floor by screw conveyor and mixed with the fermentation substrate from the digester to form a pumpable mass and pumped into the fermenter.

Pumping out the fermented substrate and the required supply of fresh substrate are controlled by automatic control of the pumps and slide valves by level gauges in the fermenter and in the gas supply system. The biogas in the gas storage tank is converted into electrical energy and heat in the CHP unit (ignition jet engine with connected generator with 80 kW continuous electrical power) with the addition of 8-10% ignition oil.

The heat emitted by the engine is primarily required for the fermenter to reach the optimum temperature of approx. 38°- 40° C for the fermentation process. Via district heating pipes, the remaining heat energy is used for heating the building. In summer, the excess heat is disposed of via backup coolers.

3. Basics and terms

The term biogas is applied to all digester gases produced in nature. If organic substances accumulate in places where there is not enough oxygen for aerobic decomposition, for example at the bottom of lakes, in swamps or even in the rumen of ruminants, such gases are produced.

Biogas itself is a mixture consisting mainly of methane (CH4), carbon dioxide (CO2), and small amounts of hydrogen sulfide (H2S) and other highly volatile gases.

Components of biogas:

- combustible methane (CH4) 50-70%

- non-combustible carbon dioxide (CO2) 30-45%

- hydrogen sulfide (H2S) 0- 1%

- hydrogen (H2) 0- 1%

- Oxygen (O2) 0- 1%

- Nitrogen (N2) 0- 1%

A suitable feedstock for the technical production of biogas is manure. On average, about 1.5 m³ of liquid manure is produced per livestock unit per month. However, this amount depends on the animal species, the age of the animals, the feedstuffs used, the cleaning regime and other factors.

From the manure produced per livestock unit and day, 1.5 m³ of biogas can be generated. A gas quantity of 1 m³ biogas corresponds on average to about 0.6 l fuel oil or 0.66 m³ natural gas.

With an average lower heating value of biogas of 6.5 kWh/m³, about 2 kWh of electrical energy can be generated.

The biogas yield can be increased by adding further organic substances (coferments).

Coferments are:

- renewable raw materials

- agricultural residues

- waste from the food industry

Most biogas plants operate according to the flow-through method. Fresh feedstock is added to the digester on a daily basis, while the same amount is removed from the fermentation tank.

The following prove to be advantageous

- a consistent gas production

- high digester utilization

- low-cost construction

- low heat losses

In the fermentation process, the building blocks of organic matter are broken down in a biochemical process in four steps by different, interdependent groups of bacteria.

1. hydrolysis takes place at the beginning; solid substances go into solution. The basic biological building blocks (proteins, fats, carbohydrates) are broken down into monomers.
2. these compounds are degraded by fermentative bacteria. Alcohol and carboxylic acids are formed (acidification).
3. these are degraded to hydrogen, carbon dioxide and acetic acid. (acetogenesis)
4. methane-forming bacteria break down acetic acid or reduce carbon dioxide using hydrogen, forming methane (methanogenesis). The volume of gas produced is highly dependent on substrate composition, substrate concentration, digestion temperature and retention time.