Biogas plants

What is biogas?

Biogas is produced as a result of anaerobic digestion of biomass. It consists of methane (СН₄) by 45-75% and carbon dioxide (СО₂) by 25-45%. Additionally, it contains small amounts of hydrogen sulfide (Н₂S), ammonia (NH₃), hydrogen (H₂) and carbon monoxide (CO).

A range of raw materials could be utilized to produce biogas, including:

1. agricultural waste, e.g. manure, by-products of harvesting crops and garden waste;

2. by-products of food industry, e.g. fruit residue, malt or hop dust, distillery dregs, sugar beet pulp, molasses, waste fat from butchering, food waste of canning factories;

3. municipal waste, e.g. organic fraction of MSW (Municipal Solid Waste), waste water, dumps and landfills;

4. energy-rich vegetation, e.g. maize and grass silage and grains.

The comparative chart below shows biogas output from some forms of substrate (m3/t of raw material):

  • Liquid pigs’ manure
  • Liquid cattle’s manure
  • Whey
  • Grain stillage
  • Dry poultry manure
  • Butchering waste (hydrolyzed)
  • Silage of whole plants of cereal crops (wheat)
  • Grass silage
  • Maize silage
  • Grains (wheat)

Main functional elements of a biogas plant:

A biogas plant is a complex of technological equipment allowing to process organic agricultural waste, food and municipal waste by means of anaerobic digestion. Biogas and sludge are generated as a result of such processing. Biogas is then transformed into heat and electricity in CHP module. Fermented substrates or sludge serve as a valuable organic fertilizer.

Name of component Brief description
1 Reception pit (CALIX) Concrete preliminary storage tank equipped with a sinking pump, sinking propeller blade agitator and a radar sensor to assess the amount of substrate received.
2 Dosing station for solid substrates (PASCO) Automated supply of solid substrates, e.g. maize silage or beet pulp to the fermenter; carried out with a loader.
3 Fermenter (COCCUS) A container made of mass concrete with special additives. The tank is insulated and plated with corrugated iron. Heated with a system of polyethylene pipes with non-stick coating. Agitators of different types and power are applied depending on the substrate and capacity of the fermenter. The container is also equipped with double membrane gas holder and an emergency valve.
4 Horizontal fermenter; (EUCO) This type of fermenter was specifically developed to ferment substances with a high proportion of dry matter such as grass, maize and solid manure, for example. The structure of EUCO is generally similar to the COCCUS but it doesn’t have a gas holder.
5 Secondary fermenter (SULA) The container is generally similar to the fermenter. Networks, mechanisms and membranes will be realized in accordance with the raw materials and the chosen method of processing.
6 CHP module CHP module turns the energy of methane into electricity or heat. The plants work with the help of a gas generator set or gas turbine. Includes a gas treatment unit which purifies and cools off the gas before it is delivered to the engine.
7 Transformer station Provides connection with the external power networks; during the start-up it supplies power to the mechanisms and equipment; in operation mode can generate electricity into the network.
  Gas flare Gas flare burns off biogas in case of emergency if it has proved impossible to make use of it as a source of energy.
  Separator Separator divides fermented mass into solid and liquid fractions. As a rule press screw separators are employed.

Why biogas?

 

1. Environmental responsibility.
The main reason to build a biogas plant is to address the problem of farm waste storage. Fresh manure cannot be immediately used as a fertilizer on fields, therefore farms and poultry plants are forced to store it in the open air for a long time. Such storage sites pollute soil, air and ground waters. However, anaerobic fermentation of waste in a biogas plant allows to reduce its smell as well as the aggressive eroding effect while also decreasing methane and ammonia emission into the air. Sludge leaving the fermenter does not contain any pathogenic bacteria, intestinal parasites or germinable weed seeds. Thus biogas plants transform raw materials into high quality fertilizers.

2. Electricity generation.
One of the important advantages of a biogas plant is the opportunity to generate electricity. This is normally achieved with the help of cogeneration plants – gas generator sets or gas turbines whose shaft is connected to the generator. The power so produced could be used in several ways, e.g. to electrify local or remote consumers, for instance another branch of a chain, through an agreement with power stations that allows to transport electricity; or else it could be sold at a green rate.

3. Heat generation.
Cogeneration plants allow to produce both electricity and heat. The latter is delivered as hot water (with the temperature of approximately 75°C) which has circulated through the heat exchanger of the co-generator and heated up there. Waste-heat boilers enable to obtain heat in the form of water or steam with different given parameters.
This power could be utilized to heat office and service building of a company or to supply them with hot water. Otherwise, it might be of use during the manufacturing process.
It is worth mentioning that a certain amount of heat and electricity covers the need of the biogas plant itself. This mainly depends on the climate zone, season and raw materials.

4. Organic fertilizers.
Fermented substrate leaving the biogas plant is no longer hazardous and may be pumped over to the lagoon or fed in to the separator. The type of substrate will define whether it is reasonable to use one. If so, the mechanic separation system divides the remaining mass into solid and liquid fractions. The solid part – vermicompost – could serve as an environmentally friendly fertilizer. Likewise, the liquid part makes a valuable complex fertilizer.

 

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