What is Biogas?

Renewable energy from waste

Biogas is produced by processing residual waste from livestock (dung, manure and uneaten food), food production (fruit and vegetable waste, residues from meat, fish and dairy processing, brewery waste, food waste and much more) and effluents from industrial as well as municipal wastewater treatment plants. By constructing biogas power plants, agriculture assumes an important contribution to supplying energy from renewable resources as well as to the disposal of organic wastes. Digestates are produced as a by-product of biogas manufacturing, which can in turn be used as high-quality fertilizer.

The implementation of the Recycling and Waste Management Act is perfetctly accomplished through the construction of rural biogas power plants, from an economic and environmental perspective.

Components

Biogas is produced by the anaerobic digestion of organic materials in a sealed fermenter. This fermenter transforms organic materials into biogas by using methane-producing bacteria through a biologically complex process at approximately 38 - 55 degrees Celsius. More than half of the resulting gas is methane (CH₄); the rest is carbon dioxide (CO₂). A combined heat and power plant (CHP) with a generator transforms the methane gas into power and heat. Biogas is completely environmentally friendly and CO₂-neutral. The production process only generates as much CO2 as previously has been absorbed by the plants during the photosynthesis. The ecological cycle is complete.

The need for biogas

Biogas is already known for several hundred years. Methane was detected in marsh gases around 1750. The first continuous fermentation processes were realized after 1900. In the beginning of the 1970s, biogas was relevant again in Europe due to the sharp rise in energy costs (oil crisis). Since 1980, the ecological assessment and presentation, and thus the awareness of the people has resulted in a major boost in interest for biogas.

Biogas, i.e. the fermentation of organic wastes, is a way to solve environmental problems:

Greenhouse effect
Destruction of the ozone layer
Phasing out of nuclear energy
Limited duration of use of traditional fossil fuels

DEVELOPMENT PROCESS

Fermentation types

Aerobic fermentation:

with air or oxygen supply
→ (end products CO₂ and H₂O)

Anaerobic fermentation:

without air or oxygen supply
→ (end product CH₄ - biogas)

Process development for the production of biogas

Hydrolysis (liquefaction)
Decomposition of organic substances into volatile fatty acids, acetic acids, butyric acids, propionic acids (similar to silage preparation)

Conversion of these end products into methane CH₄, carbon dioxide CO₂ and water H₂O.

The fermentation process is carried out by bacteria living in a mixed culture in the fermenter.

Important influencing factors for biogas extraction

Temperature Psychrophilic 15 - 30 °C

→ long duration, low gas yield, sensitive fermentation process

Mesophilic 30 - 45 °C

→ short duration, good gas yield, stable fermentation process

Thermophilic 45 - 60 °C

→ short duration, good gas yield
→ unstable fermentation process
→ sanitation effect

Storage period

There are close relationships between the digester temperature and the storage period in the fermenter. For temperatures in the thermophilic range, the storage period can be reduced to 25 days. The optimal storage period for the process is about 40 - 60 days

Solids content

The optimum is between 4% - 12% of organic dry solids (ODS).Under 4% ODS, the energy content of the digested substrate is too low.Over 12% ODS, the digested substrate can no longer be pumped, it has limited flow properties.

Fermentable substances

a) Farm fertilizer from agricultural cattle manure

Solid cattle manure
Pig slurry
Solid pig manure
Chicken slurry
Dry chicken manure

b) Agricultural residues from grass clippings

Beet leaves
Silage

c) Residues from the agro-industry spent grains

Fruit waste
Vegetable waste
Rapeseed meal
Grain residue
Dregs (potatoes, distillery etc.)
Molasses

d) Municipal slaughterhouse waste (rumina contents, flotate fat)

Grease separator
Food waste
Catering industry wastes
Domestic sewage

Generally, during the approval process, it must be ascertained that sanitation measures have been prescribed for certain areas of the latter group.

Biogas production from the anaerobic fermentation of various substrates

Substrate	Liters of gas / kg of ODS
Cattle manure	300 - 400
Pig manure	400 - 700
Chicken manure (thinned)	400 - 700
Organic household waste	450
Food waste	1.000
Vegetable waste	600
Green waste	600
Grease from grease separators	1.000

Process

Slurry tank

The slurry tank serves as interim storage and a collecting container for liquid fertilizer. The size of the slurry tank must be sufficient for 2-3 days of manure and substrate accumulation. Reserves are beneficial. In the slurry tank, rough amounts of solids such as stones, which may not enter into the fermenter, can settle.

The merging of small amounts of solids and the feeding of the fermenter are carried out with a robust pump. Generally, an additional mixer is available in the slurry tannk to mix and to avoid floating layers. In some cases, it is possible to renounce the slurry tank and to introduce the manure directly into the fermenter.

Slurry tank with hydrolysis level

For plant systems with hydrolysis, the extruder introduces pulped material into the first slurry tank through the feed screw. The first slurry tank is the so-called hydrolysis stage and therefore the first stage of the two-stage fermentation. In the hydrolysis slurry tank, the molecules are first split with the help of microorganisms in two phases (hydrolysis and acidogenesis). This generates the fission products; hydrogen, carbon dioxide, alcohols and fatty acids. The pulped material can now be processed faster and easier in the fermenter. Substrate is pumped into the fermenter several times per day from the hydrolysis stage.

In order to maintain a constant temperature, the hydrolysis slurry tank is heated by waste heat from the CHP and is insulated outside with Styrodur.

Acceptance dispenser / Biofeeder

Optionally, depending on the initial biomass, either an acceptance dispenser with a downstream extruder (BIOaccelerator^s) or a biofeeder with a downstream impact reactor (BIOacceleratorr) is used for pre-treatment.

The acceptance dispenser consists of an open container with an integrated scraper floor and milling drums. From the acceptance dispenser, the material passes through milling rollers and conveyors to enter the BIOaccelerator^s. In the BIOaccelerator^r the material passes from the biofeeder through a screw system. (Exception: Farmer series).

The biofeeder also consists of an open container and transports the material using a so-called "Walking Floor". From the biofeeder, the material passes through milling drums to reach the BIOaccelerator^r.

Fermenter

The fermenter is the heart of the plant. This is where the actual conversion of biomass to biogas (fermentation) takes place. Using different microorganisms, the organic substrate is decomposed and transformed into methane-rich biogas. This is the established single-stage fermentation process in the mesophilic range. This means that the entire conversion process takes place in a single container at a temperature between 35 – 45°C. As the microorganisms cannot maintain this temperature level by themselves, a heater is integrated into the fermenter. This heater consists of heating pipes, which are attached to the inner wall of the fermenter in ring shapes (similar to a radiator). Here, some of the waste heat from the CHP acts as a heat source.

The fermenter is continuously fed by a "flow-through" process. This means that fresh substrate is introduced into the fermenter from the slurry tank via the feed screw, several times per day. A sufficiently large mechanical mixer circulating the fermentation substrate is critical for optimum gas yield. Pneumatic and hydraulic agitation technologies have not been successful in practice. When adding certain substrates, floating and sinking layers can be formed in the fermenter. The mixer must therefore be height-adjustable.Likewise, substrate is removed from the fermenter and pumped into the post-fermenter/final storage, several times per day. This is a proven method and ensures constant gas production as well as a very good utilization of the fermenter volume. The fermenter itself is manufactured on-site using reinforced concrete. It is insulated with Styrodur to reduce heat loss. The part of the container protruding from the ground is covered with corrugated metal or wood. The cover of the fermenter is designed as a carrier air roof. Here, the foil roof itself stores gas.

Pump station

The distribution of liquids between the containers is carried out via the central pumping station. The pump house with its central pumping station is positioned between the fermenter and post-fermenter. Generally, it is possible to pump from any container in the plant into any other container.

Heat distribution is also located in the pump house. The pump station can also be delivered in a container.

Gas preparation / Gas storage

Biogas contains sulfur components which attack and destroy non-ferrous metals in the storage area of the combined heating and power plants. Therefore, the biogas must be de-sulfurized in the fermenter through

oxidation using iron
Air entry of 2 - 3 Vol % via small compressors or aquarium pumps.

The latter solution has proven itself to be the most functional and economical.The pipelines for conducting gas to the CHP are of a large dimension (approx. 100–150 mm, depending on distance). PVC or PE pipes are used in the ground and stainless steel pipes are used in open installation areas. Pipelines laid in the ground for longer periods of time have the benefit that the gas is cooled and therefore the water vapor is condensed and thereby separated. The cooler the gas the better its quality. Gas lines must be installed along a slope. Gas storage occurs in PVC or PE foil containers welded on-site according to the low-pressure principal. The storage volumes should be designed for 6–12 hours. This allows for overnight gas storage and the coverage of higher power needs during the day.

CHP with transformer

The CHP is operated using the biogas from the fermenter and, to a lesser extent, the biogas from the secondary fermenter/final storage.

The electrical energy produced in the CHP is fed into the municipal power grid and a small portion is consumed by the plant itself. A part of the thermal energy from the motor cooling and the flue gas heat exchanger is used to heat the fermenter and the secondary fermenter. The excess heat is used to dry the digestate. If insufficient heat is removed, the emergency CHP coolers are used to cool the motor.