The use of biogas in everyday life. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Alternative energy sources

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For many, the first question that arises is how to match existing needs with the size of the installation. So many write: the size of the house, say, 5x6 m (or the volume, for example, 150 m3), the family is 4 people, it is necessary to heat up and provide a kitchen; What size installation is required?

Experience shows that, on average, heating a house with an area of ​​40-50 m2 and a four-burner stove requires 3,0-3,5 m3 of biogas per hour. When equipping a local heating system, you can use the widely used automatic heating gas water heater AOGV-11, 3-3-U.

An important factor determining the intensity of gas formation is the temperature of the process. It should not be forgotten that the article "Biogas: both heats and cooks" describes an experience related to a country with a fairly mild climate. Apparently, for more severe climatic conditions, heating is more necessary, perhaps even in a steady process. And if heating is envisaged, then it seems appropriate to use it as an effective regulatory factor, due to which it is possible to increase gas generation several times. (We will talk about another controlling factor - mixing - below.)

Now, taking into account the joint influence of these factors on the capacity of the installation, we can give some recommendations.

When choosing the size of the fermenter, you can focus on the options given in the last publication; taking into account a more severe climate, it is worth adding a heating element to the installation, for example, in the form of coils. Trial operation will immediately reveal the effect of heating on the performance of the device. To systematize the finishing work, it is recommended to have a notebook (not relying on memory) and write down all the changes - both introduced and received. Practice shows that every 10° additional heating of the biomass doubles the gas yield from 1 m3 of the fermenter.

Here is some data for those who are going to design the installation. From 1 ton of raw materials, 80-100 m2 of gas is obtained. Its calorific value is approximately 5500-6000 kcal/m3. For comparison: household gas is not much more caloric - only 7000 kcal / m3.

Now about the biology of the process. Methane-producing bacteria are present in the raw material itself. Their cultures develop in the fermenter for up to three weeks, until the mass begins to release gas. When using a ready-made "sourdough" from a previous portion from an already operating fermenter, the gas production start time is reduced to about a week.

Methane-producing bacteria are divided into three groups. Psychrophilic ones work effectively in the range +5...+20°. With a further increase in temperature, mesophilic bacteria develop, their working range is +30...+42°. And at an even higher temperature, the action of already thermophilic bacteria is manifested, which work in a very narrow range: +54 ... + 56 °.

A large number of questions relate to the design of the installation, first of all, to the creation of the possibility of periodic refueling of raw materials and mixing of biomass without depressurization of the bell. First of all, it must be said that continuous gas production can be obtained by duplicating installations. With two fermenters, when refilling them in turn, it is possible to do without complicating the design.

Therefore, the future creator of a plant for the production of biogas should compare, in relation to their capabilities, three schemes: the simplest with periodic refueling; paired protozoa, with alternate refueling; with a special device that provides a continuous supply of gas.

When choosing the third scheme, it must be borne in mind that the operation of the fermenter requires not only refueling with raw materials, but also the removal of waste.

In the latter scheme, refueling of raw materials and waste disposal are not equivalent in frequency. Thus, waste disposal can be combined with a process stop for cleaning and system revision. As for refueling, it is done more often and is easier: 1/10 of the volume is removed daily from the bottom and the same amount of fresh bio-raw materials is added from above.

One of the possible ways to top up the fermenter without losing gas is based on the so-called principle of communicating vessels. To do this, a small filling tank is arranged next to the fermenter pit, connected to it by a pipeline located below the liquid level (Fig. 1). The pipeline is made from a piece of ceramic sewer or asbestos-cement pipe, which is embedded in the walls of containers. Such a system is itself a liquid gas seal. It is possible to increase the efficiency of concentrate supply using an insert funnel-bunker (Fig. 1a). It is possible to push the thick through the pipeline with the simplest mesh piston. At the same time, it is also used as a damper that prevents self-mixing of biomass between both containers.

Fig. 1

Many questions are raised by the need for periodic mixing of biomass. How to perform this operation without depressurization? Not everyone knows about the possibility of its self-mixing. Recall the effect of convection: it can be observed in a room when some fluff is above the radiator, floats up, sinks against the opposite wall and is again carried away by the air flow to the radiator. This effect of the thermal circulation of the medium can also be easily obtained in the fermenter if heating pipes (coil) are placed in its lower part, shifting them to one edge; convection will provide self-mixing. In the process of gas formation that has begun, the effect of the rise of gas bubbles in the zone located above the heater will be added to this.

It is not difficult to make a mechanical biomass stirrer. It is especially appropriate in areas with a mild climate, where there is no need to use heating. As practice shows, it is better to foresee this in advance. After all, if the system itself starts heating, then why, one wonders, waste energy on mixing. In addition, it is not at all necessary to mix the mass continuously. You can do this periodically, for example, in the morning and in the evening. It is even worth turning this operation into an additional, adjusting one. To do this, it is enough to follow the position of the bell: as soon as it drops to the lower level (small gas supply), you need to mix the biomass - and the gas release will immediately increase.

The simplest mixer is easy to manufacture in the form of an impeller driven by flexible connections through the same siphon pipeline (Fig. 3). There is no need for continuous rotation in one direction. If the agitator has radial blades, oscillating movements are sufficient. You can limit yourself to one blade (Fig. 2). In general, there is room for your own decisions. It is better to use non-rotting materials as rods, for example, an insulated electrical wire or a nylon (chloride) cord, sold in hardware stores as underwear.

Fig. 2

Fig. 3

There is also the problem of the stability of the bell. Readers who have carefully studied the material "Biogas: both heats and cooks" have already noticed that if the schemes shown in Figure 1 are carried out without modifying the structure, then the bell may lose balance as soon as it floats up: it will either capsize or jam. In figure 3 in the same publication, it is not by chance that a guide tube for the bell is provided, but such an installation is more difficult to make at home.

In the figure, we show the balancing scheme of the bell with two blocks (Fig. 4a) and a counterweight and the "crane" variant (Fig. 4b). The error resulting from the non-strict vertical movement of the bell's suspension point on the "crane" (along the arc of a circle) is negligible due to the significant excess of the lever arm over the rocker travel.

Fig. 4

Such a bell balancing system is also advantageous in that it can be used as a lifting device when inspecting and cleaning the fermenter. Given this, it is not difficult to supplement the design with some auxiliary elements: it is better to place the blocks on a second boom (after all, it is strictly not allowed to lift the bell to work under it - "Do not stand under the load!"). It is worth making a rotary and support of the "crane" rocker, and the counterweight is type-setting, as on warehouse scales. But if there is no frost in your area, provide a counterweight in the form of a container filled with water.

But the most serious difficulty that stands in the way of a do-it-yourselfer is the manufacture of a bell. Galvanized roofing iron allows you to give it the desired shape with simple means, besides, it will not be heavy. But the fragility of such a material with rapid corrosion in an aggressive environment forces us to look for other options. Therefore, we strongly advise you to take a closer look at the available scrap metal. Old containers, for example, from oil products, when cut, can be a very suitable semi-finished product, both in shape (usually with welded spherical bottoms) and in sheet material thickness: from 2 to 5 mm.

Apparently, the running dimensions of the bell will be Ø 2-3 m and the same height. If the "barrel" turns out to be smaller, it is worth considering whether to make a large bell or take two smaller ones (for example, Ø 1,5 m), at the same time returning to the option of paired simple installations.

Some readers have a question about determining the pressure of a gas. Apparently, they did not pay attention to the obvious: as soon as the bell floats, the gas pressure force has reached the value of the mass of the bell. Let's explain this with an example. With a bell skirt diameter of 2 m, its cross-sectional area will be S=πR2=3,14*1=3,14 m2=31 cm400. With a bell wall thickness of 2 mm and a height of 5 m, its weight will be about 2 kg. Assume that the actual weight of the bell is 500 kg. Then the bell will float at a gas pressure of 470 atm. (In the SI system, mass M = 0,15 kg, weight force G = 470 N, gas pressure p = 4700/4700 = 31 N/cm400 = 0,15 atm).

As the bell rises, the pressure will hardly change, its increase will occur only due to the displacement of a volume of liquid equal to the part of the bell walls that has surfaced.

Noting the low gas pressure, we see that it (if necessary) can be increased in a simple way: install an additional weight on the bell, placing it lower, for better balance of the bell.

A few interesting examples for comparison. The gas pressure in the city network is in the range of 200-300 mm of water. Art., and allowed - up to 600 mm of water. Art. In our system, this pressure should also be limiting. Naturally, the question arises: is a personal farmstead able to provide bio-raw materials in sufficient quantities? Of course not. Our recommendations apply primarily to cooperative livestock farms, which are getting more and more developed every day. In addition, reserves, and considerable ones, lie in collective farms and state farms: sometimes a significant amount of manure accumulates near livestock farms, which is not used in any way. Local residents could dispose of it, and then take it to the fields. After all, the waste raw material from the fermenter practically does not lose its value as a fertilizer. There is a double economic benefit.

In conclusion, we again appeal to readers with a request to share their experience in the design and operation of biogas plants.

Author: P.Zak

See other articles Section Alternative energy sources.

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