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Installations for biogas

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Installations for biogas

What is biogas?
In recent years, non-traditional, from a technical point of view, energy sources are attracting more and more attention: solar radiation, tides and waves, and much more. Some of them, such as wind, have found wide application in the past, and today they are experiencing a rebirth. One of the “forgotten” types of raw materials is biogas, which was used in ancient China and re-opened in modern times.
What is biogas? This term refers to the gaseous product obtained as a result of anaerobic, that is, occurring without air access, fermentation (overheating) of organic substances of the most diverse origin. In any farm during the year a significant amount of manure, tops of plants, and various wastes is collected. Usually, after decomposition, they are used as organic fertilizer. However, few people know how much biogas and heat is released during fermentation. But this energy can also serve the villagers in good stead.

Biogas - a mixture of gases. Its main components: methane (CH4) - 55-70% and carbon dioxide (CO2) - 28-43%, and also in very small quantities other gases, for example - hydrogen sulfide (H2S).
On average, 1 kg of 70% bio-degradable organic matter produces 0.18 kg of methane, 0.32 kg of carbon dioxide, 0.2 kg of water and 0.3 kg of non-decomposable residue.
Factors affecting biogas production.
Since the decomposition of organic waste occurs due to the activity of certain types of bacteria, the environment has a significant impact on it. Thus, the amount of produced gas largely depends on temperature: the warmer, the higher the rate and degree of fermentation of organic raw materials. That is why, probably, the first biogas plants appeared in countries with a warm climate. However, the use of reliable thermal insulation, and sometimes heated water allows you to master the construction of biogas generators in areas where the temperature in winter drops to -20? There are certain requirements for raw materials: it must be suitable for the development of bacteria, contain biodegradable organic matter and a large amount of water (90-94%). It is desirable that the environment be neutral and free of substances that interfere with the action of bacteria: for example, soap, laundry detergents, antibiotics.

To obtain biogas, plant and household waste, manure, wastewater, etc. can be used. In the process of fermentation, the liquid in the tank tends to be divided into three fractions. The upper crust, formed from large particles entrained by rising gas bubbles, may become hard enough after some time and will interfere with the release of biogas. Liquid accumulates in the middle part of the fermenter, and the lower, mud-like fraction precipitates.
Bacteria are most active in the middle zone. Therefore, the contents of the tank must be periodically mixed - at least once a day, and preferably up to six times. Mixing can be carried out using mechanical devices, hydraulic means (recirculation under the action of the pump), under the pressure of the pneumatic system (partial recycling of biogas) or using various methods of self-mixing.
Installations for biogas.
In Romania, biogas generators are widespread. One of the first individual plants (Fig. 1A) was commissioned in December 1982. Since then, she has been successfully supplying gas to three neighboring families, each with an ordinary gas stove with three burners and an oven. The fermenter is located in a pit with a diameter of about 4 m and a depth of 2 m (volume about 21 m3), lined from the inside with roofing iron, welded twice: first with electric welding, and then, for reliability, with gas. For corrosion protection, the internal surface of the tank is coated with resin. Outside the upper edge of the fermenter, an annular groove was made of concrete about 1 m deep that performs the function of a hydraulic lock; in this groove filled with water, the vertical portion of the bell that slides the tank slides.
The bell, about 2.5 m high, is made of two-millimeter sheet steel. In the upper part of the gas is collected.
The author of this project chose the option of collecting gas in contrast to other installations using a pipe located inside the fermenter and having three underground branches - to three farms. In addition, the water in the groove of the water seal is flowing, which prevents icing in the winter. The fermenter loads approximately 12 m3 of fresh manure, over which cow urine is poured (without adding water. The generator starts to work 7 days after filling.

Another installation has a similar layout (Fig. 1B). Its fermenter is made in a pit that has a square cross-section of 2x2 and a depth of about 2.5 m. The pit is lined with reinforced concrete slabs 10-12 cm thick, plastered with cement and coated with resin for sealing. The groove of the water seal with a depth of about 50 cm is also concrete, the bell is welded from roofing iron and can slide on the four “ears” freely along the four vertical guides mounted on the concrete tank. The height of the bell is about 3 m, of which 0.5 m is immersed in the groove.
At the first filling, 8 m3 of fresh cow dung was loaded into the fermenter, and about 400 liters of cow urine were washed from above. After 7-8 days, the installation already fully supplied the owners with gas.

A biogas generator designed to receive 6 m3 of mixed manure (from cows, sheep and pigs) has a similar design. This was enough to ensure the normal operation of a gas stove with three burners and an oven.

Another installation is distinguished by any constructive detail: next to the fermenter, three large tractor chambers attached to it with the help of a T-shaped hose, connected to each other (rig. 2). At night, when biogas is not used and accumulates under the bell, there is a danger that the latter will overturn due to excessive pressure. The rubber tank serves as an additional capacity. A 2x2x1.5 m fermenter is quite sufficient for the operation of two burners, and with an increase in the useful volume of the installation up to 1 m3 it is possible to obtain a quantity of biogas sufficient to heat the dwelling.
The peculiarity of this installation option is the device of a bell of 138 cm and a height of 150 cm from rubberized cloth used for the manufacture of inflatable rails. The fermenter is a metal tank 140x380 cm and has a volume of 4.7 m3. The bell is introduced into the manure in the fermenter to a depth of at least 30 cm to provide a hydraulic barrier to the exit of biogas into the atmosphere. A tap is connected to the top of the swellable reservoir; gas flows through it to a gas stove with three burners and a column for heating water. To ensure optimal conditions for the operation of the fermenter, the manure is mixed with hot water.
The installation showed the best results at a raw material humidity of 90% and a temperature of 30-35 °.

The greenhouse effect is also used to heat the fermenter. A metal frame is constructed above the tank, which is covered with plastic wrap: under adverse weather conditions, it retains heat and allows you to significantly speed up the process of decomposition of raw materials.

In Romania, biogas generators are also used in state or cooperative farms. Here is one of them. It has two fermenters with a capacity of 203 m3 each, covered with a frame with plastic wrap (Fig. 3). In winter, manure is heated with hot water. The capacity of the installation is 300-480 m3 of gas per day. This amount is enough to meet all the needs of the local agro-industrial complex.
Practical advice.
As already noted, the decisive role. in the development of the fermentation process, the temperature plays: heating the raw material with 15? up to 20 ° can double the production of energy. Therefore, generators often have a special system for heating raw materials, but most installations are not equipped with it; they use only the heat generated in the process of the decomposition of organic substances. One of the most important conditions for the normal operation of the fermenter is the presence of reliable THERMAL INSULATION. In addition, it is necessary to minimize heat loss during cleaning and filling of the fermenter bin.

It is also necessary to remember about the need to ensure biochemical equilibrium. Sometimes the rate of bacterial production of acids is higher than the rate of their consumption by bacteria of the second group. In this case, the acidity of the mass increases, and the production of biogas decreases. The situation can be corrected either by decreasing the daily portion of the raw material, or by increasing its solubility (if possible with hot water), or, finally, by adding a neutralizing agent - for example, milk of lime, washing or baking soda.

Biogas production can be reduced by disrupting the ratio between carbon and nitrogen. In this case, nitrogen-containing substances — urine or ammonium salts in a small amount — usually used as chemical fertilizers (50-100 g per 1 m3 of raw materials) are introduced into the fermenter.

It should be remembered that high humidity and the presence of hydrogen sulfide (the content of which in biogas can reach 0.5%) stimulate increased corrosion of the metal parts of the installation. Therefore, the condition of all other elements of the fermenter should be regularly monitored and carefully protected in places of damage: it is best to use lead lead in one or two layers, and then two more layers of any oil paint.

As a pipeline for transporting biogas from the outlet in the upper part of the installation bell to the consumer, both pipes (metal or plastic) and rubber hoses can be used. It is advisable to keep them in a deep trench in order to exclude breaks due to freezing of condensed water in winter. If the gas is transported by means of a hose through the air, then a special device is needed to drain the condensate.

The simplest scheme of such a device is a U-shaped tube connected to the hose at its lowest point (Fig. 4). The length of the free tube branch (x) must be greater than the biogas pressure expressed in millimeters of a water column. As the condensate drains from the pipeline into the tube, water is poured out through its free end without gas leakage.
In the upper part of the bell, it is also advisable to provide a nozzle for the installation of the pressure gauge to judge the amount of accumulated biogas by pressure.

The operating experience of the plants has shown that the use of a mixture of various organic substances as raw materials gives more biogas than when one of the components is loaded with a fermenter. The moisture content of the raw materials is recommended to be slightly reduced in winter (up to 88-90%) and increased in summer (92-94%). Water used for dilution should be warm (preferably 35-40 °).

Raw materials are served in portions, at least once a day. After the first loading of the fermenter, it is often first produced biogas, which contains more than 60% carbon dioxide and therefore does not burn. This gas is removed to the atmosphere, and after 1-3 days the installation will begin to function normally.