DIY biogas plant

Rising energy prices make us think about the possibility of self-sufficiency. One option is a biogas plant. With its help, biogas is obtained from manure, litter and plant residues, which, after cleaning, can be used for gas appliances (stove, boiler), pumped into cylinders and used as fuel for cars or electric generators. In general, the processing of manure into biogas can provide all the energy needs of a home or farm. 

General Principles

Biogas is a product that is obtained from the decomposition of organic matter. In the process of decay / fermentation, gases are released, by collecting which you can meet the needs of your own household. The equipment in which this process takes place is called a “biogas plant”.

In some cases, the gas output is excessive, then it is stored in gas tanks – for use during the period of its insufficient quantity. With proper organization of the gas process, there may be too much gas, then its surplus can be sold. Another source of income is fermented leftovers. This is a highly effective and safe fertilizer – in the process of fermentation, most microorganisms die, plant seeds lose their germination capacity, parasite eggs become unviable. The export of such fertilizers to the fields has a positive effect on productivity.

Conditions for gas production

The process of biogas formation occurs due to the vital activity of various kinds of bacteria that are contained in the waste itself. But in order for them to actively “work”, they need to create certain conditions: humidity and temperature. To create them, a biogas plant is being built. This is a complex of devices, the basis of which is a bioreactor, in which the decomposition of waste occurs, which is accompanied by gas formation.

There are three modes of processing manure into biogas:

• Psychophilic mode. The temperature in the biogas plant is from +5°C to +20°C. Under such conditions, the decomposition process is slow, a lot of gas is formed, its quality is low.
• Mesophilic. The unit enters this mode at temperatures from +30°C to +40°C. In this case, mesophilic bacteria actively multiply. In this case, more gas is formed, the processing process takes less time – from 10 to 20 days.
• Thermophilic. These bacteria multiply at temperatures above +50°C. The process is the fastest (3-5 days), the gas yield is the largest (under ideal conditions, up to 1 liters of gas can be obtained from 4,5 kg of delivery). Most reference tables for gas yield from processing are given specifically for this mode, so when using other modes, it is worth making an adjustment down.

The most difficult thing in biogas plants is the thermophilic regime. This requires high-quality thermal insulation of a biogas plant, heating and a temperature control system. But at the output we get the maximum amount of biogas. Another feature of thermophilic processing is the impossibility of reloading. The remaining two modes – psychophilic and mesophilic – allow you to add a fresh portion of prepared raw materials daily. But, in the thermophilic mode, a short processing time makes it possible to divide the bioreactor into zones in which its share of raw materials with different loading times will be processed.

Scheme of a biogas plant

The basis of a biogas plant is a bioreactor or bunker. The fermentation process takes place in it, and the resulting gas accumulates in it. There is also a loading and unloading bunker, the generated gas is discharged through a pipe inserted into the upper part. Next comes the gas refinement system – its cleaning and increasing the pressure in the gas pipeline to the working one.

For mesophilic and thermophilic regimes, a bioreactor heating system is also required to reach the required regimes. For this, gas-fired boilers are usually used. From it, the pipeline system goes to the bioreactor. Usually these are polymer pipes, as they best tolerate being in an aggressive environment.

Another biogas plant needs a system for mixing the substance. During fermentation, a hard crust forms at the top, heavy particles settle down. All this together worsens the process of gas formation. To maintain a homogeneous state of the processed mass, agitators are necessary. They can be mechanical or even manual. Can be started by timer or manually. It all depends on how the biogas plant is made. An automated system is more expensive to install, but requires a minimum of attention during operation.

Biogas plant by type of location can be:

• Overhead.
• Semi-submerged.
• Buried.

More expensive to install buried – a large amount of land work is required. But when operating in our conditions, they are better – it is easier to organize insulation, less heating costs.

What can be recycled

A biogas plant is essentially omnivorous – any organic matter can be processed. Any manure and urine, plant residues are suitable. Detergents, antibiotics, chemicals negatively affect the process. It is desirable to minimize their intake, as they kill the flora that is involved in processing.

Cattle manure is considered ideal, as it contains microorganisms in large quantities. If there are no cows in the farm, when loading the bioreactor, it is desirable to add some of the litter to populate the substrate with the required microflora. Plant residues are pre-crushed, diluted with water. In the bioreactor, vegetable raw materials and excrement are mixed. Such a “refueling” takes longer to process, but at the exit, with the right mode, we have the highest product yield.

Positioning

To minimize the cost of organizing the process, it makes sense to locate a biogas plant near the source of waste – near buildings where birds or animals are kept. It is desirable to develop a design so that loading occurs by gravity. From a cowshed or pigsty, a pipeline can be laid under a slope, through which manure will flow by gravity into the bunker. This greatly simplifies the task of maintaining the reactor, and cleaning up manure too.

Usually buildings with animals are located at some distance from a residential building. Therefore, the generated gas will need to be transferred to consumers. But stretching one gas pipe is cheaper and easier than organizing a line for transporting and loading manure.

Bioreactor

Quite stringent requirements are imposed on the manure processing tank:

• It must be impervious to water and gases. Watertightness must work both ways: the liquid from the bioreactor must not pollute the soil, and groundwater must not change the state of the fermented mass.
• The bioreactor must have high strength. It must withstand the mass of a semi-liquid substrate, the gas pressure inside the container, and the soil pressure acting from the outside. In general, when building a bioreactor, special attention must be paid to its strength.
  

• Serviceability. More convenient to use cylindrical containers – horizontal or vertical. In them, mixing can be organized throughout the volume; stagnant zones do not form in them. Rectangular containers are easier to implement when building with your own hands, but cracks often form in their corners, and the substrate stagnates there. It is very problematic to mix it in the corners. 

All these requirements for the construction of a biogas plant must be met, as they ensure safety and create normal conditions for the processing of manure into biogas.

What materials can be made

Resistance to aggressive environments is the main requirement for materials from which containers can be made. The substrate in the bioreactor may be acidic or alkaline. Accordingly, the material from which the container is made must be well tolerated by various media.

Not many materials answer these requests. The first thing that comes to mind is metal. It is durable, it can be used to make a container of any shape. What’s good is that you can use a ready-made container – some kind of old tank. In this case, the construction of a biogas plant will take very little time. The lack of metal is that it reacts with chemically active substances and begins to break down. To neutralize this minus, the metal is covered with a protective coating.

An excellent option is the capacity of a polymer bioreactor. Plastic is chemically neutral, does not rot, does not rust. Only it is necessary to choose from such materials that endure freezing and heating to sufficiently high temperatures. The walls of the reactor should be thick, preferably reinforced with fiberglass. Such containers are not cheap, but they last a long time.

A cheaper option is a biogas plant with a tank made of bricks, concrete blocks, stone. In order for the masonry to withstand high loads, it is necessary to reinforce the masonry (in each 3-5 row, depending on the wall thickness and material). After completion of the wall erection process, subsequent multilayer treatment of the walls, both inside and outside, is necessary to ensure water and gas impermeability. The walls are plastered with a cement-sand composition with additives (additives) that provide the required properties.

Reactor sizing

The volume of the reactor depends on the selected temperature for processing manure into biogas. Most often, mesophilic is chosen – it is easier to maintain and it implies the possibility of daily additional loading of the reactor. Biogas production after reaching the normal mode (about 2 days) is stable, without bursts and dips (when normal conditions are created). In this case, it makes sense to calculate the volume of the biogas plant depending on the amount of manure generated on the farm per day. Everything is easily calculated based on the average data.

Decomposition of manure at mesophilic temperatures takes from 10 to 20 days. Accordingly, the volume is calculated by multiplying by 10 or 20. When calculating, it is necessary to take into account the amount of water that is necessary to bring the substrate to an ideal state – its humidity should be 85-90%. The found volume is increased by 50%, since the maximum load should not exceed 2/3 of the volume of the tank – gas should accumulate under the ceiling.

For example, the farm has 5 cows, 10 pigs and 40 chickens. As a matter of fact, 5 * 55 kg + 10 * 4,5 kg + 40 * 0,17 kg = 275 kg + 45 kg + 6,8 kg = 326,8 kg are formed. To bring chicken manure to a moisture content of 85%, you need to add a little more than 5 liters of water (that’s another 5 kg). The total mass is 331,8 kg. For processing in 20 days it is necessary: ​​331,8 kg * 20 u6636d 7 kg – about 1,5 cubes only for the substrate. We multiply the found figure by 50 (increase by 10,5%), we get XNUMX cubic meters. This will be the calculated value of the volume of the biogas plant reactor.

Loading and unloading

Loading and unloading hatches lead directly to the bioreactor tank. In order for the substrate to be evenly distributed over the entire area, they are made at opposite ends of the container.

With the buried installation method of the biogas plant, the loading and unloading pipes approach the body at an acute angle. Moreover, the lower end of the pipe should be below the liquid level in the reactor. This prevents air from entering the container. Also, rotary or shut-off valves are installed on the pipes, which are closed in the normal position. They are only open for loading or unloading.

Since the manure may contain large fragments (bedding elements, grass stalks, etc.), small diameter pipes will often become clogged. Therefore, for loading and unloading, they must be 20-30 cm in diameter. They must be installed before the start of work on the insulation of the biogas plant, but after the container is installed in place.

The most convenient mode of operation of a biogas plant is with regular loading and unloading of the substrate. This operation can be performed once a day or once every two days. Manure and other components are pre-collected in a storage tank, where they are brought to the required state – crushed, if necessary, moistened and mixed. For convenience, this container may have a mechanical stirrer. The prepared substrate is poured into the receiving hatch. If you place the receiving container in the sun, the substrate will be preheated, which will reduce the cost of maintaining the required temperature.

It is desirable to calculate the installation depth of the receiving hopper so that the waste flows into it by gravity. The same applies to unloading into the bioreactor. The best case is if the prepared substrate moves by gravity. And a damper will block it off during the preparation.

To ensure the tightness of the biogas plant, hatches on the receiving hopper and in the unloading area must have a sealing rubber seal. The less air there is in the tank, the cleaner the gas will be at the outlet.

Collection and disposal of biogas

The removal of biogas from the reactor occurs through a pipe, one end of which is under the roof, the other is usually lowered into a water seal. This is a container with water into which the resulting biogas is discharged. There is a second pipe in the water seal – it is located above the liquid level. More pure biogas comes out into it. A shut-off gas valve is installed at the outlet of their bioreactor. The best option is ball.

What materials can be used for the gas transmission system? Galvanized metal pipes and gas pipes made of HDPE or PPR. They must ensure tightness, seams and joints are checked with soap suds. The entire pipeline is assembled from pipes and fittings of the same diameter. No contractions or expansions.

Purification of impurities

The approximate composition of the resulting biogas is as follows:

• methane — up to 60%;
• carbon dioxide — 35%;
• other gaseous substances (including hydrogen sulfide, which gives the gas an unpleasant odor) – 5%.

In order for biogas to have no smell and burn well, it is necessary to remove carbon dioxide, hydrogen sulfide, and water vapor from it. Carbon dioxide is removed in a water seal if slaked lime is added to the bottom of the installation. Such a bookmark will have to be changed periodically (as the gas starts to burn worse, it’s time to change it).

Gas dehydration can be done in two ways – by making hydraulic seals in the gas pipeline – by inserting curved sections under the hydraulic seals into the pipe, in which condensate will accumulate. The disadvantage of this method is the need for regular emptying of the water seal – with a large amount of collected water, it can block the passage of gas.

The second way is to put a filter with silica gel. The principle is the same as in the water seal – the gas is fed into the silica gel, dried out from under the cover. With this method of drying biogas, silica gel has to be dried periodically. To do this, it needs to be warmed up for some time in the microwave. It heats up, the moisture evaporates. You can fall asleep and use again.

To remove hydrogen sulfide, a filter loaded with metal shavings is used. You can load old metal washcloths into the container. Purification occurs in exactly the same way: gas is supplied to the lower part of the container filled with metal. Passing, it is cleaned of hydrogen sulfide, collects in the upper free part of the filter, from where it is discharged through another pipe / hose.

Gas tank and compressor

The purified biogas enters the storage tank – gas tank. It can be a sealed plastic bag, a plastic container. The main condition is gas tightness, the shape and material do not matter. Biogas is stored in the gas tank. From it, with the help of a compressor, gas under a certain pressure (set by the compressor) is already supplied to the consumer – to a gas stove or boiler. This gas can also be used to generate electricity using a generator.

To create a stable pressure in the system after the compressor, it is desirable to install a receiver – a small device for leveling pressure surges.

Mixing devices

In order for the biogas plant to operate normally, it is necessary to regularly mix the liquid in the bioreactor. This simple process solves many problems:

• mixes a fresh portion of the load with a colony of bacteria;
• promotes the release of the produced gas;
• equalizes the temperature of the liquid, excluding warmer and colder areas;
• maintains the homogeneity of the substrate, preventing the settling or surfacing of some constituents.

Typically, a small homemade biogas plant has mechanical agitators that are driven by muscle power. In systems with a large volume, the agitators can be driven by motors that are switched on by a timer.

The second way is to mix the liquid by passing through it part of the generated gas. To do this, after leaving the metatank, a tee is placed and part of the gas is poured into the lower part of the reactor, where it exits through a tube with holes. This part of the gas cannot be considered a consumption, since it still enters the system again and, as a result, ends up in the gas tank.

The third mixing method is to pump the substrate from the lower part with the help of fecal pumps, pour it out at the top. The disadvantage of this method is the dependence on the availability of electricity.

Heating system and thermal insulation

Without heating the processed slurry, psychophilic bacteria will multiply. The processing process in this case will take from 30 days, and the gas yield will be small. In summer, in the presence of thermal insulation and preheating of the load, it is possible to reach temperatures up to 40 degrees, when the development of mesophilic bacteria begins, but in winter such an installation is practically inoperable – the processes are very sluggish. At temperatures below +5°C, they practically freeze.

What to heat and where to place

Heat is used for best results. The most rational is water heating from the boiler. The boiler can operate on electricity, solid or liquid fuel, it can also be run on the generated biogas. The maximum temperature to which water must be heated is +60°C. Hotter pipes can cause particles to adhere to the surface, resulting in reduced heating efficiency.

A biogas plant can be heated using standard heating radiators, simply pipes twisted into a coil, welded registers. It is better to use polymer pipes – metal-plastic or polypropylene. Corrugated stainless steel pipes are also suitable, they are easier to lay, especially in cylindrical vertical bioreactors, but the corrugated surface provokes sediment build-up, which is not very good for heat transfer.

To reduce the possibility of deposition of particles on the heating elements, they are placed in the stirrer zone. Only in this case it is necessary to design everything so that the mixer cannot touch the pipes. It often seems that it is better to place the heaters from below, but practice has shown that due to sediment at the bottom, such heating is inefficient. So it is more rational to place the heaters on the walls of the metatank of the biogas plant.

Water heating methods

According to the way the pipes are located, heating can be external or internal. When located indoors, heating is efficient, but repair and maintenance of heaters is impossible without shutting down and pumping out the system. Therefore, special attention is paid to the selection of materials and the quality of the connections.

When the heaters are located outdoors, more heat is required (the cost of heating the contents of a biogas plant is much higher), since a lot of heat is spent on heating the walls. But the system is always available for repair, and the heating is more uniform, since the medium is heated from the walls. Another plus of this solution is that agitators cannot damage the heating system.

How to insulate

At the bottom of the pit, first, a leveling layer of sand is poured, then a heat-insulating layer. It can be clay mixed with straw and expanded clay, slag. All these components can be mixed, can be poured in separate layers. They are leveled into the horizon, the capacity of the biogas plant is installed.

The sides of the bioreactor can be insulated with modern materials or classic old-fashioned methods. Of the old-fashioned methods – coating with clay and straw. It is applied in several layers.

From modern materials, you can use high-density extruded polystyrene foam, low-density aerated concrete blocks, foamed polyurethane foam. The most technologically advanced in this case is polyurethane foam (PPU), but the services for its application are not cheap. But it turns out seamless thermal insulation, which minimizes heating costs. There is another heat-insulating material – foamed glass. In plates, it is very expensive, but its battle or crumb costs quite a bit, and in terms of characteristics it is almost perfect: it does not absorb moisture, is not afraid of freezing, tolerates static loads well, and has low thermal conductivity.