Table of contents:
- Biogas heating - history
- Biogas - what is it
- What factors determine the yield of biogas with a higher methane content
- Biogas plants
- DIY biogas plant
- Related Videos
Video: Alternative Heating - Biogas
2023 Author: Douglas Hoggarth | [email protected]. Last modified: 2023-05-24 11:23
- Biogas heating - history
- Biogas - what is it
- What factors determine the yield of biogas with a higher methane content
- Biogas plants
DIY biogas plant
- Bioreactor shape
- Location of the bioreactor
- Bioreactor equipment
- Thermal insulation of the bioreactor
- Loading and unloading of organic substrate
- How to collect biogas
- Gas filter
- Shut-off valves and valves
- Mechanical stirring
- Heating the organic substrate in the bioreactor
- Gas tanks for biogas collection
- Related Videos
Among the important components of our life, energy carriers are of great importance, the prices of which are growing almost every month. Each winter season makes a dent in family budgets, forcing them to bear heating costs, which means fuel for heating boilers and stoves. But what to do, because electricity, gas, coal or firewood cost money, and the further our homes are from major energy mains, the more expensive it will be to heat them. Meanwhile, alternative heating, independent of any suppliers and tariffs, can be built on biogas, the production of which does not require geological exploration, drilling of wells, or expensive pumping equipment.
Biogas can be obtained practically at home, while incurring minimal, quickly recouped costs - you will find a lot of information on this issue in our article.
Biogas heating - history
Interest in the combustible gas formed in swamps during the warm season of the year arose even among our distant ancestors - the advanced cultures of India, China, Persia and Assyria experimented with biogas over 3 millennia ago. In the same ancient times in tribal Europe, the Alemanni Swabians noticed that the gas released in the swamps burns perfectly - they used it to heat their huts, supplying gas to them through leather pipes and burning them in the hearths. The Swabians considered biogas to be the "breath of dragons" who, in their opinion, lived in swamps.
Centuries and millennia later, biogas experienced its second discovery - in the 17th and 18th centuries, two European scientists at once paid attention to it. The famous chemist of his time, Jan Baptista van Helmont, established that a combustible gas is formed during the decomposition of any biomass, and the famous physicist and chemist Alessandro Volta established a direct relationship between the amount of biomass in which decomposition processes take place and the amount of biogas released. In 1804, the English chemist John Dalton discovered the formula for methane, and four years later the Englishman Humphrey Davy discovered it in a swamp gas.
Left: Jan Baptista van Helmont. Right: Alessandro Volta
Interest in the practical application of biogas arose with the development of gas street lighting - at the end of the 19th century, the streets of one district of the English city of Exeter were lit with gas obtained from a sewer.
In the 20th century, the need for energy resources caused by the Second World War forced Europeans to look for alternative energy sources. Biogas plants, in which gas was produced from manure, spread in Germany and France, partly in Eastern Europe. However, after the victory of the countries of the anti-Hitler coalition, they forgot about biogas - electricity, natural gas and oil products fully covered the needs of industries and the population.
In the USSR, biogas production technology was considered mainly from an academic point of view and was not considered in any way in demand.
Today, the attitude towards alternative energy sources has changed dramatically - they have become interesting, since the cost of conventional energy sources increases from year to year. At its core, biogas is a real way to get away from tariffs and costs for classic energy sources, to get your own source of fuel, and for any purpose and in sufficient quantity.
The largest number of biogas plants have been created and are in operation in China: 40 million units of medium and small capacity, the volume of methane produced is about 27 billion m 3 per year.
Biogas - what is it
It is a gas mixture, consisting mainly of methane (content from 50 to 85%), carbon dioxide (content from 15 to 50%) and other gases in a much lower percentage. Biogas is produced by a team of three types of bacteria that feed on biomass - hydrolysis bacteria that produce food for acid-forming bacteria, which in turn supply food for methane-producing bacteria that form biogas.
Biogas chemical composition
Fermentation of the original organic material (for example, manure), the product of which will be biogas, takes place without access to the external atmosphere and is called anaerobic. Another product of such fermentation, called compost humus, is well known to the villagers who use it to fertilize fields and vegetable gardens, but biogas and heat energy produced in compost heaps are usually not used - and in vain!
What factors determine the yield of biogas with a higher methane content
First of all - from the temperature. The activity of bacteria fermenting organic matter is the higher, the higher the temperature of their environment; at subzero temperatures, fermentation slows down or stops completely. For this reason, biogas production is most common in Africa and Asia, located in the subtropics and tropics. In the Russian climate, the production of biogas and a complete transition to it as an alternative fuel will require thermal insulation of the bioreactor and the introduction of warm water into the mass of organic matter when the temperature of the external atmosphere drops below zero.
The organic material put into the bioreactor must be biodegradable; it is required to introduce a significant amount of water into it - up to 90% of the organic mass. An important point will be the neutrality of the organic environment, the absence in its composition of components that prevent the development of bacteria, such as cleaning and detergents, any antibiotics. Biogas can be obtained from almost any waste of household and plant origin, sewage, manure, etc.
The process of anaerobic fermentation of organics works best when the pH value is in the range of 6.8-8.0 - high acidity will slow down the formation of biogas, since bacteria will be busy consuming acids and producing carbon dioxide, which neutralizes acidity.
The ratio of nitrogen and carbon in the bioreactor must be calculated as 1 to 30 - in this case, the bacteria will receive the amount of carbon dioxide they need, and the methane content in the biogas will be the highest.
The best yield of biogas with a sufficiently high methane content is achieved if the temperature in the fermented organic matter is in the range of 32–35 ° C; at lower and higher values, the content of carbon dioxide increases in biogas, its quality decreases. Bacteria that produce methane are divided into three groups: psychrophilic, effective at temperatures from +5 to +20 ° C; mesophilic, their temperature regime is from +30 to +42 ° С; thermophilic, operating in the mode from +54 to +56 ° С. For the biogas consumer, mesophilic and thermophilic bacteria are of the greatest interest, fermenting organic matter with a greater gas yield.
Mesophilic fermentation is less sensitive to changes in temperature by a couple of degrees from the optimal temperature range, requires less energy to heat the organic material in the bioreactor. Its disadvantages, in comparison with thermophilic fermentation, are lower gas yield, longer period of complete processing of the organic substrate (about 25 days), the resulting organic material decomposed may contain harmful flora, since the low temperature in the bioreactor does not provide 100% sterility.
Raising and maintaining the in-reactor temperature at a level acceptable for thermophilic bacteria will provide the highest biogas yield, complete fermentation of organic matter will take place in 12 days, and the decomposition products of the organic substrate are completely sterile. Negative characteristics: going beyond the temperature range acceptable for thermophilic bacteria by 2 degrees will lower the gas yield; high demand for heating, as a result - significant energy costs.
The contents of the bioreactor must be stirred at intervals of 2 times a day, otherwise a crust forms on its surface, which creates a barrier for biogas. In addition to eliminating it, stirring allows you to equalize the temperature and acidity level inside the organic mass.
In bioreactors of a continuous cycle, the greatest biogas yield occurs with the simultaneous unloading of fermented organic matter and loading of new organic matter in an amount equal to the discharged volume. In small bioreactors, which are usually used in summer cottages, it is necessary to extract and add organic matter every day in a volume equal to approximately 5% of the internal volume of the fermentation chamber.
The biogas yield directly depends on the type of organic substrate put into the bioreactor (below are the average data per kg of dry substrate weight):
- horse manure gives 0.27 m 3 of biogas, methane content 57%;
- cattle manure (cattle) gives 0.3 m 3 of biogas, the methane content is 65%;
- fresh cattle manure gives 0.05 m 3 of biogas with 68% methane content;
- chicken droppings - 0.5 m 3, the methane content in it will be 60%;
- pig manure - 0.57 m 3, the share of methane will be 70%;
- sheep manure - 0.6 m 3 with a methane content of 70%;
- wheat straw - 0.27 m 3, with 58% methane content;
- corn straw - 0.45 m 3, methane content 58%;
- grass - 0.55 m 3, with 70% methane content;
- woody foliage - 0.27 m 3, the proportion of methane is 58%;
- fat - 1.3 m 3, methane content 88%.
These devices consist of the following main elements - a reactor, a bunker for loading organic matter, a biogas outlet, a bunker for unloading fermented organic matter.
By design type, biogas plants are of the following types:
- without heating and without stirring the fermented organic matter in the reactor;
- without heating, but with stirring of organic matter;
- with heating and stirring;
- with heating, stirring and devices allowing to control and manage the fermentation process.
A biogas plant of the first type is suitable for a small farm and is designed for psychrophilic bacteria: the internal volume of the bioreactor is 1–10 m 3 (processing 50–200 kg of manure per day), the minimum configuration, the resulting biogas is not stored - it immediately goes to household appliances that consume it. This unit can only be used in the southern regions, it is designed for an internal temperature of 5–20 ° C. Fermented organic matter is removed simultaneously with the loading of a new batch; the shipment is carried out into a container, the volume of which must be equal to or greater than the internal volume of the bioreactor. The contents of the container are stored in it until they are introduced into the fertilized soil.
The construction of the second type is also designed for a small farm, its productivity is slightly higher than the biogas plants of the first type - the equipment includes a mixing device with manual or mechanical drive.
The third type of biogas plants is equipped, in addition to the mixing device, with forced heating of the bioreactor, while the hot water boiler operates on an alternative fuel produced by the biogas plant. Mesophilic and thermophilic bacteria are engaged in the production of methane in such installations, depending on the intensity of heating and the temperature level in the reactor.
Schematic diagram of a biogas plant: 1 - substrate heating; 2 - filler neck; 3 - capacity of the bioreactor; 4 - hand stirrer; 5 - container for collecting condensate; 6 - gas valve; 7 - tank for processed mass; 8 - safety valve; 9 - filter; 10 - gas boiler; 11 - gas valve; 12 - gas consumers; 13 - odor seal
The last type of biogas plants is the most complex and is designed for several consumers of biogas, an electric contact pressure gauge, a safety valve, a hot-water boiler, a compressor (pneumatic mixing of organic matter), a receiver, a gas holder, a gas reducer, a branch for loading biogas into transport are introduced into the design of the plants. These installations operate continuously, they allow the setting of any of three temperature regimes thanks to precisely adjustable heating, biogas sampling is carried out automatically.
DIY biogas plant
The calorific value of biogas produced in biogas plants is approximately equal to 5,500 kcal / m 3, which is slightly lower than the calorific value of natural gas (7,000 kcal / m 3). Heating 50 m 2 of a residential building and using a gas stove with four burners will require an average of 4 m 3 of biogas per hour.
Industrial plants for the production of biogas offered on the Russian market cost from 200,000 rubles. - with their outwardly high cost, it is worth noting that these installations are accurately calculated in terms of the volume of loaded organic substrate and they are covered by manufacturers' warranties.
If you want to create a biogas plant yourself, then further information is for you!
The best shape for it will be oval (egg-shaped), but it is extremely difficult to build such a reactor. It will be easier to design a bioreactor of cylindrical shape, the upper and lower parts of which are made in the form of a cone or semicircle. Reactors of a square or rectangular shape made of brick or concrete will be ineffective, since cracks will form in the corners in them over time, caused by the pressure of the substrate, they will also accumulate solidified fragments of organic matter that interfere with the fermentation process.
Steel tanks of bioreactors are hermetically sealed, resistant to high pressure, and not so difficult to build. Their disadvantage is that they are poorly resistant to rust; it is required to apply a protective coating, for example, resin, to the inner walls. The outside surfaces of the steel bioreactor must be thoroughly cleaned and painted in two layers.
Containers of bioreactors made of concrete, brick or stone must be carefully coated from the inside with a layer of resin, capable of ensuring their effective water and gas tightness, withstanding temperatures of about 60 ° C, the aggression of hydrogen sulfide and organic acids. In addition to resin, to protect the internal surfaces of the reactor, you can use paraffin, diluted with 4% engine oil (new) or kerosene and heated to 120–150 ° C - the surfaces of the bioreactor must be heated with a burner before applying a paraffin layer on them.
When creating a bioreactor, you can use rust-free plastic containers, but only from rigid ones with sufficiently strong walls. Soft plastic can be used only in the warm season, since with the onset of cold weather it will be difficult to fix the insulation on it, besides, its walls are not strong enough. Plastic bioreactors can only be used for psychrophilic fermentation of organic matter.
Location of the bioreactor
Its placement is planned depending on the free space on the site, the distance from residential buildings, the place of disposal of waste and animals, etc. The planning of a ground-based, fully or partially submerged bioreactor depends on the level of groundwater, the convenience of input and output of the organic substrate into the container reactor. It will be optimal to place the reactor vessel below ground level - savings on equipment for the introduction of an organic substrate are achieved, thermal insulation is significantly increased, for which inexpensive materials (straw, clay) can be used.
The reactor tank needs to be equipped with a hatch, with the help of which repair and maintenance work can be performed. A rubber gasket or sealant must be placed between the bioreactor body and the manhole cover. Equipping the bioreactor with a sensor for temperature, internal pressure and organic substrate level will be optional, but extremely convenient.
Thermal insulation of the bioreactor
Its absence will not allow the biogas plant to operate all year round, only in warm weather. Clay, straw, dry manure and slag are used to insulate a buried or semi-buried bioreactor. The insulation is laid in layers - when installing a buried reactor, the foundation pit is covered with a layer of PVC film, which prevents direct contact of the thermal insulation material with the soil. Before installing the bioreactor, straw is poured onto the bottom of the pit, a layer of clay is placed on top of it, then the bioreactor is exposed. After that, all the free areas between the reactor vessel and the pit laid by PVC-film are filled with straw almost to the end of the vessel, a 300 mm layer of clay mixed with slag is poured on top.
Loading and unloading of organic substrate
The diameter of the pipes for loading into and discharging from the bioreactor must be at least 300 mm, otherwise they will clog. In order to maintain anaerobic conditions inside the reactor, each of them should be equipped with screw or semi-turn valves. The volume of the bunker for the supply of organic matter, depending on the type of biogas plant, must be equal to the daily volume of the input raw material. The feed hopper should be located on the sunny side of the bioreactor, as this will increase the temperature in the introduced organic substrate, speeding up the fermentation process. If the biogas plant is connected directly to the farm, then the bunker should be placed under its structure so that the organic substrate enters it under the influence of gravity.
The pipelines for loading and unloading the organic substrate should be located on opposite sides of the bioreactor - in this case, the injected raw material will be evenly distributed, and the fermented organic matter will be easily extracted under the influence of gravitational forces and the mass of fresh substrate. Holes and installation of the pipeline for loading and unloading organic matter should be performed before installing the bioreactor at the installation site and before placing layers of thermal insulation on it. The tightness of the internal volume of the bioreactor is achieved by the fact that the pipe entries are located at an acute angle, while the liquid level inside the reactor is higher than the pipe entry points - a hydraulic seal blocks air access.
The introduction of new and the withdrawal of fermented organic material is easiest to carry out according to the overflow principle, that is, raising the level of organic matter inside the reactor when a new portion is introduced will remove the substrate through the discharge pipe in a volume equal to the volume of the introduced material.
If fast loading of organic matter is required, and the efficiency of material introduction by gravity is low due to lack of relief, installation of pumps will be required. There are two ways: dry, in which the pump is installed inside the loading pipe and organic matter, coming to the pump through a vertical pipe, is pumped by it; wet, in which the pump is installed in the loading hopper, its drive is carried out by a motor also installed in the hopper (in an impermeable housing) or through a shaft, while the motor is installed outside the hopper.
How to collect biogas
This system includes a gas pipeline that distributes gas to consumers, shut-off valves, tanks for collecting condensate, a safety valve, a receiver, a compressor, a gas filter, a gas holder and gas consumption devices. The installation of the system is carried out only after the complete installation of the bioreactor at the location.
The outlet for collecting biogas is carried out at the highest point of the reactor, it is connected in series: a sealed container for collecting condensate; safety valve and water seal - a container with water, the gas pipeline inlet into which is made below the water level, the outlet is above (the gas pipeline in front of the water seal should be bent so that water does not penetrate into the reactor), which will not allow gas to move in the opposite direction.
The biogas formed during the fermentation of the organic substrate contains a significant amount of water vapor, which forms condensate along the walls of the gas pipeline and, in some cases, blocking the flow of gas to consumers. Since it is difficult to build a gas pipeline in such a way that along its entire length there is a slope towards the reactor, where the condensate would drain, then in each of its low sections it is required to install water gates in the form of containers with water. During operation of a biogas plant, it is periodically required to remove part of the water from them, otherwise its level will completely block the flow of gas.
The gas pipeline must be built with pipes of the same diameter and of the same type; all valves and system elements must also have the same diameter. Steel pipes with a diameter of 12 to 18 mm are applicable for biogas plants of small and medium capacity, the consumption of biogas supplied through pipes of these diameters should not exceed 1 m 3 / h (at a flow rate of 0.5 m 3 / h, the use of pipes with a diameter 12 mm for a length over 60 m). The same condition applies when plastic pipes are used in the gas pipeline, in addition, these pipes must be laid below ground level by 250 mm, since their plastic is sensitive to sunlight and loses strength under the influence of solar radiation.
When laying a gas pipeline, it is necessary to carefully make sure that there are no leaks and gas tightness of the joints - the check is performed with soapy water.
Biogas contains a small amount of hydrogen sulfide, the combination of which with water creates an acid that actively corrodes the metal - for this reason, unfiltered biogas cannot be used for internal combustion engines. Meanwhile, hydrogen sulfide can be removed from the gas with a simple filter - a 300 mm piece of gas pipe filled with a dry mixture of metal and wood chips. After every 2,000 m 3 of biogas passed through such a filter, it is necessary to extract its contents and hold for about an hour in the open air - the chips will be completely purified from sulfur and can be reused.
Shut-off valves and valves
In the immediate vicinity of the bioreactor, a main gas valve is installed, a valve that releases biogas at a pressure of more than 0.5 kg / cm 2 should be cut into the gas pipeline. The best taps for a gas system are chrome-plated ball valves; taps designed for plumbing systems cannot be used in a gas system. At each of the gas consumers, the installation of a ball valve is required.
For bioreactors with a small volume, manual agitators are best suited - they are simple in design and do not require any special conditions during operation. A mechanically driven agitator is designed as follows - a horizontal or vertical shaft located inside the reactor along its central axis, blades are fixed on it, moving the masses of organic matter rich in bacteria from the unloading site of the fermented substrate to the place of loading a fresh portion during rotation. Be careful - the mixer should only rotate in the direction of mixing from the unloading area to the loading area, the movement of methane-forming bacteria from the matured substrate to the newly supplied one will accelerate the maturation of organic matter and the production of biogas with a high methane content.
How often should the organic substrate be stirred in the bioreactor? It is necessary to determine the frequency by observation, focusing on the output of biogas - excessively frequent stirring will disrupt fermentation, since it will interfere with the activity of bacteria, in addition, will cause the withdrawal of unprocessed organic matter. On average, the time interval between stirring should be from 4 to 6 hours.
Heating the organic substrate in the bioreactor
Without heating, the reactor can produce biogas only in a psychrophilic mode, as a result, the amount of gas produced will be less, and the quality of fertilizers is worse than under higher-temperature mesophilic and thermophilic operating modes. Heating of the substrate can be done in two ways: heating with steam; combining organic matter with hot water or heating with a heat exchanger in which hot water circulates (without mixing with organic material).
A serious drawback of steam heating (direct heating) is the need to include a steam generation system in the biogas plant, which includes a water purification system from the salt present in it. A steam generation plant is only beneficial for really large plants handling large volumes of substrate, eg waste water. In addition, heating with steam will not allow you to accurately control the heating temperature of the organic matter, as a result, it may overheat.
Heat exchangers located inside or outside the bioreactor unit indirectly heat the organic matter inside the reactor. It is worth immediately discarding the option with heating through the floor (foundation), since the accumulation of solid sediment at the bottom of the bioreactor prevents it. The best option would be to introduce the heat exchanger into the reactor, however, the material forming it must be strong enough and successfully withstand the pressure of organic matter when it is stirred. A heat exchanger with a larger area will heat the organic matter better and more uniformly, thereby improving the fermentation process. External heating, with its lower efficiency due to heat loss from the walls, is attractive in that nothing inside the bioreactor will interfere with the movement of the substrate.
The optimum temperature in the heat exchanger should be about 60 ° C, the heat exchangers themselves are made in the form of radiator sections, coils, parallel welded pipes. Maintaining the temperature of the coolant at 60 ° C will reduce the threat of suspension particles sticking to the walls of the heat exchanger, the accumulation of which will significantly reduce heat transfer. The optimal location for the heat exchanger is near the stirring blades, in this case the threat of organic particles settling on its surface is minimal.
The heating pipeline of the bioreactor is designed and equipped similarly to a conventional heating system, that is, the conditions for returning chilled water to the lowest point of the system must be observed, air release valves at its upper points are required. The temperature control of the organic mass inside the bioreactor is carried out with a thermometer, with which the reactor should be equipped.
Gas tanks for biogas collection
With constant gas consumption, there is no need for them, except that they can be used to equalize the gas pressure, which will significantly improve the combustion process. For bioreactor installations of small capacity, automobile chambers of a large volume, which can be connected in parallel, are suitable for the role of gasholders.
More serious gasholders, steel or plastic, are selected for a specific bioreactor plant - in the best version, the gasholder should contain the volume of biogas produced daily. The required capacity of the gasholder depends on its type and the pressure for which it is designed, as a rule, its volume is 1/5 … 1/3 of the internal volume of the bioreactor.
Steel gas tank. There are three types of steel gas tanks: low pressure, from 0.01 to 0.05 kg / cm 2; medium, from 8 to 10 kg / cm 2; high, up to 200 kg / cm 2. It is impractical to use steel low-pressure gas tanks, it is better to replace them with plastic gas tanks - they are expensive and are applicable only with a significant distance between the biogas plant and consumer devices. Low pressure gas tanks are mainly used to equalize the difference between the daily biogas output and its actual consumption.
Biogas is pumped into steel gas tanks of medium and high pressure by a compressor; they are used only in bioreactors of medium and large capacity.
Gas tanks must be equipped with the following instrumentation: a safety valve, a water seal, a pressure reducer and a pressure gauge. Gas tanks made of steel must be grounded!