Biogas plant: recycling organic waste profitably

Recommendation points

• How biogas is obtained
• What raw materials are suitable for production
• Bioreactor construction
• Gas removal and enrichment
• Biogas use: specificity and equipment

The rise in energy prices is forcing the search for alternative heating options. Good results can be achieved by self-production of biogas from available organic raw materials. In this article we will talk about the production cycle, bioreactor device and related equipment..

Subject to basic operating rules, a gas reactor is completely safe and capable of providing fuel and electricity to even a small house, even an entire agro-industrial complex. The result of the bioreactor’s work is not only gas, but also one of the most valuable types of fertilizers, the main component of natural humus.

How biogas is obtained

To obtain biogas, organic raw materials are placed in conditions favorable for the development of several types of bacteria, which emit methane during their vital activity. Biomass goes through three transformation cycles, and at each stage different strains of anaerobic organisms take part. Oxygen is not required for their vital functions, but the composition of the raw material and its consistency, as well as temperature and internal pressure, are of great importance. Conditions with a temperature of 40–60 ° C at a pressure of up to 0.05 atm are considered optimal. The loaded raw material begins to produce gas after prolonged activation, which takes from several weeks to six months.

The beginning of the gas release in the calculated volume indicates that the bacterial colonies are already quite numerous, therefore, after 1–2 weeks, fresh raw material is dosed into the reactor, which is activated almost immediately and enters the production cycle.

To maintain optimal conditions, the raw materials are periodically mixed, using part of the heat from gas heating to maintain the temperature. The resulting gas contains from 30 to 80% methane, 15-50% carbon dioxide, small impurities of nitrogen, hydrogen and hydrogen sulfide. For use in the household, gas is enriched by removing carbon dioxide from it, after which the fuel can be used in a wide range of power equipment: from engines of power plants to heating boilers..

What raw materials are suitable for production

Contrary to popular belief, manure is not the best raw material for biogas production. Fuel output from a ton of clean manure is only 50-70 m³ with a concentration of 28-30%. However, it is in animal waste that most of the necessary bacteria are found to quickly start up and maintain efficient operation of the reactor..

For this reason, manure is mixed with crop and food waste in a ratio of 1: 3. The following are used as plant raw materials:

Raw materials	Output from 1 ton of raw materials	СН concentration4
Silage of corn stalks and cobs	400 m3	50-56%
Grass and grain silage	200-230 m3	49-54%
Fodder potatoes, beets, grain	500-600 m3	50-65%
Waste from bakeries and food industry (soybeans, oats)	700-750 m3	55-58%
Vegetable oils, fat, glycerin	8500-1200 m3	65-68%

Raw materials cannot simply be poured into the reactor, some preparation is needed. The original substrate is crushed to a fraction of 0.4–0.7 mm and diluted with water in an amount of about 25–30% of the dry weight. In large volumes, the mixture requires more thorough mixing in homogenizing devices, after which it is ready for loading into the reactor.

Bioreactor construction

The requirements for the conditions for placing the reactor are the same as for a passive septic tank. The main part of the bioreactor is the digester – a container in which the entire fermentation process takes place. To reduce the cost of heating the mass, the reactor is dug into the ground. Thus, the temperature of the medium does not drop below 12–16 ° С, and the heat outflow formed during the reaction remains minimal.

Biogas plant diagram: 1 – raw material loading bunker; 2 – biogas; 3 – biomass; 4 – tank compensator; 5 – hatch for waste extraction; 6 – pressure relief valve; 7 – gas tube; 8 – water seal; 9 – to consumers

For digesters up to 3 m³ it is allowed to use nylon containers. Since the thickness and material of their walls do not interfere with the outflow of heat, the containers are covered with layers of expanded polystyrene or moisture-resistant mineral wool. The bottom of the pit is concreted with a screed of 7-10 cm with reinforcement to prevent the reactor from being squeezed out of the ground.

The most suitable material for the construction of large reactors is reinforced claydite concrete. It has sufficient strength, low thermal conductivity and a long service life. Before pouring the walls of the chamber, it is necessary to mount an inclined pipe to supply the mixture to the reactor. Its diameter is 200-350 mm, the lower end should be 20-30 cm from the bottom.

In the upper part of the digester there is a gas tank – a dome or conical structure that concentrates gas at the top point. The gas holder can be made of sheet metal, however, in small installations, the arch is made with brickwork, and then covered with steel mesh and plastered. When constructing a gas tank, it is necessary to provide in its upper part a sealed passage of two pipes: for gas intake and installation of a pressure relief valve. Another pipe with a diameter of 50-70 mm is laid to pump out the waste mass.

The reactor vessel must be sealed and withstand a pressure of 0.1 atm. To do this, the inner surface of the digester is covered with a continuous layer of coated bitumen waterproofing, and a sealed hatch is mounted on the top of the gas tank..

Gas removal and enrichment

From under the dome of the gasholder, gas is diverted through a pipeline into a container with a water seal. The thickness of the water layer above the tube outlet determines the operating pressure in the reactor and is usually 250–400 mm.

After the water seal, the gas can be used in heating equipment and for cooking. However, for the operation of internal combustion engines, a higher methane content is needed, so the gas is enriched.

The first stage of enrichment is to reduce the concentration of carbon dioxide in the gas. To do this, you can use special equipment operating on the principle of chemical absorption or on semi-permeable membranes. At home, enrichment is also possible by passing gas through the water column, in which up to half of CO is dissolved₂. The gas is sprayed into small bubbles through tubular aerators, the carbonated water must be periodically removed and sprayed under normal atmospheric conditions. In plant growing complexes, such water is successfully used in hydroponic systems.

At the second stage of enrichment, the moisture content of the gas is reduced. This feature is found in most of the factory made dressing equipment. Homemade desiccants look like a Z-tube filled with silica gel.

Biogas use: specificity and equipment

Most modern models of heating equipment are designed to work with biogas. Outdated boilers can be relatively easily re-equipped by replacing the burner and air-gas preparation device.

To obtain gas under operating pressure, a conventional reciprocating compressor with a receiver is used, set to operate at a pressure of 1.2 times the design pressure. The pressure is normalized by a gas regulator, which helps to avoid drops and maintain an even flame.

Bioreactor performance must be at least 50% higher than consumption. No surplus gas is generated in production: when the pressure exceeds 0.05–0.065 atm, the reaction is almost completely slowed down, and is restored only after part of the gas has been pumped out.