Biofuel. Composition of raw materials and parameters of its processing. Encyclopedia of radio electronics and electrical engineering

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microbiology

The production of biogas and biofertilizers from organic waste is based on the property of waste to release biogas when decomposed in anaerobic, i.e. anoxic conditions. This process is called methane digestion and occurs in three stages as a result of the decomposition of organic matter by two main groups of microorganisms - acidic and methane.

Three stages of biogas production

The biogas production process can be divided into three stages: hydrolysis, oxidation and methane generation. This complex complex of transformations involves many microorganisms, the main of which are methane-producing bacteria, three types of which are shown in Fig. 8.

Fig.8. Three types of methane bacteria. Source: AT Information: Biogas, GTZ project Information and Advisory Service on Appropriate Technology (ISAT), Eshborn, Deutschland, 1996

Hydrolysis

In the first step, (hydrolysis), organic matter is fermented externally by extracellular enzymes (fiber, amylase, protease and lipase) of microorganisms. Bacteria decompose long chains of complex hydrocarbons, proteins and lipids into shorter chains.

Fermentation

Acid-producing bacteria, which take part in the second stage of biogas formation, break down complex organic compounds (fiber, proteins, fats, etc.) into simpler ones. At the same time, primary fermentation products appear in the fermented medium - volatile fatty acids, lower alcohols, hydrogen, carbon monoxide, acetic and formic acids, etc. These organic substances are a food source for methane-forming bacteria that convert organic acids into biogas.

Methane generation

The methane-producing bacteria involved in the third step decompose the low molecular weight formations. They utilize hydrogen, carbon dioxide and acetic acid. Under natural conditions, methane-producing bacteria exist in the presence of anaerobic conditions, for example, under water, in swamps. They are very sensitive to environmental changes, therefore, the intensity of gas evolution depends on the conditions that are created for the life of methane-forming bacteria.

Symbiosis of bacteria

Methano- and acid-forming bacteria interact in symbiosis. On the one hand, acid-producing bacteria create an atmosphere with ideal parameters for methane-producing bacteria (anaerobic conditions, low molecular weight chemical structures). On the other hand, methane-producing microorganisms use the intermediate compounds of acid-producing bacteria. If this interaction did not occur, unsuitable conditions for the activity of both types of microorganisms would develop in the reactor.

Parameters and optimization of the fermentation process

Acid-forming and methane-forming bacteria are ubiquitous in nature, in particular in animal excrement. For example, the digestive system of cattle contains a complete set of microorganisms necessary for the fermentation of manure, and the process of methane fermentation itself begins in the intestines. Therefore, cattle manure is often used as a raw material loaded into a new reactor, where the following conditions are sufficient to start the fermentation process:

• Maintaining anaerobic conditions in the reactor;
• Compliance with the temperature regime;
• Availability of nutrients for bacteria;
• Choosing the right fermentation time and timely loading and unloading of raw materials;
• Compliance with acid-base balance;
• Compliance with the ratio of carbon and nitrogen;
• Selection of the correct moisture content of raw materials;
• Regular stirring;
• No process inhibitors.

Each of the different types of bacteria involved in the three stages of methane formation are affected differently by these parameters. There is also a strong interdependence between the parameters (for example, the timing of digestion depends on the temperature regime), so it is difficult to determine the exact influence of each factor on the amount of biogas produced.

Maintenance of anaerobic conditions in the reactor

The vital activity of methane-forming bacteria is possible only in the absence of oxygen in the reactor of a biogas plant, therefore, it is necessary to monitor the tightness of the reactor and the lack of access to oxygen in the reactor.

Compliance with temperature conditions

Temperature range of the fermentation process

Maintaining the optimum temperature is one of the most important factors in the fermentation process. Under natural conditions, the formation of biogas occurs at temperatures from 0°C to 97°C, but taking into account the optimization of the process of processing organic waste to produce biogas and biofertilizers, 3 temperature regimes are distinguished:

• Psychophilic temperature regime is determined by temperatures up to 20 - 25°C;
• The mesophilic temperature regime is determined by temperatures from 25°C to 40°C;
• The thermophilic temperature regime is determined by temperatures above 40°C.

Minimum average temperature

The degree of bacteriological production of methane increases with increasing temperature. But, since the amount of free ammonia also increases with increasing temperature, the fermentation process may slow down. On average, biogas plants without reactor heating show satisfactory performance only when the average annual temperature is about 20°C or higher, or when the average daily temperature reaches at least 18°C. At average temperatures of 20-28°C, gas production increases disproportionately. If the temperature of the biomass is less than 15°C, the gas output will be so low that a biogas plant without thermal insulation and heating is no longer economically viable8.

Optimum raw material temperature

Information regarding the optimal temperature regime is different for different types of raw materials, but based on the empirical data of the Fluid PF installations operating in Kyrgyzstan on mixed manure of cattle, pigs and birds, the optimal temperature for the mesophilic temperature regime is 36 - 38 ° C, and for the thermophilic 52 - 55 ° C. The psychophilic temperature regime is observed in installations without heating, in which there is no temperature control. The most intense release of biogas in psychophilic mode occurs at 23°C.

Raw material temperature changes

The biomethanation process is very sensitive to temperature changes. The degree of this sensitivity, in turn, depends on the temperature range in which the processing of raw materials takes place. During the fermentation process, temperature changes within the limits of:

• Psychophilic temperature regime: 2°C per hour;
• Mesophilic temperature regime: 1°C per hour;
• Thermophilic temperature regime: 0,5°C per hour.

Thermophilic or mesophilic mode?

The advantages of the thermophilic digestion process include: an increased rate of decomposition of the raw material and, consequently, a higher yield of biogas, as well as the almost complete destruction of pathogenic bacteria contained in the raw material.

The disadvantages of thermophilic decomposition are: a large amount of energy required to heat the raw material in the reactor, the sensitivity of the digestion process to minimal temperature changes and a slightly lower quality of the resulting biofertilizers.

In the mesophilic mode of fermentation, a high amino acid composition of biofertilizers is preserved, but the disinfection of raw materials is not as complete as in the thermophilic mode.

Nutrients

For the growth and vital activity of methane bacteria, the presence of organic and mineral nutrients in the raw material is necessary. In addition to carbon and hydrogen, the creation of biofertilizers requires a sufficient amount of nitrogen, sulfur, phosphorus, potassium, calcium and magnesium and a certain amount of trace elements - iron, manganese, molybdenum, zinc, cobalt, selenium, tungsten, nickel and others. The usual organic raw material - animal manure contains a sufficient amount of the above elements.

Fermentation time

The optimal digestion time depends on the reactor loading dose and the temperature of the digestion process. If the fermentation time is chosen too short, when the digested biomass is discharged, the bacteria are washed out of the reactor faster than they can multiply and the fermentation process practically stops. Too long exposure of raw materials in the reactor does not meet the objectives of obtaining the largest amount of biogas and biofertilizers for a certain period of time.

Reactor turnaround time

When determining the optimal duration of fermentation, the term "reactor turnover time" is used. The reactor turnaround time is the time during which fresh feed loaded into the reactor is processed and discharged from the reactor.

For systems with continuous loading, the average digestion time is determined by the ratio of the volume of the reactor to the daily volume of feedstock. In practice, the turnaround time of the reactor is chosen depending on the fermentation temperature and the composition of the feedstock in the following intervals:

• Psychophilic temperature regime: from 30 to 40 or more days;
• Mesophilic temperature regime: from 10 to 20 days;
• Thermophilic temperature regime: from 5 to 10 days.

Daily dose of loading of raw materials

The daily dose of loading of raw materials is determined by the turnaround time of the reactor and increases with increasing temperature in the reactor. If the reactor turnover time is 10 days, then the daily share of the load will be 1/10 of the total volume of the loaded raw material. If the turnaround time of the reactor is 20 days, then the daily share of the load will be 1/20 of the total volume of the loaded raw material. For plants operating in thermophilic mode, the load fraction can be up to 1/S of the total reactor load.

Raw material processing time

The choice of fermentation time also depends on the type of raw material being processed. For the following types of raw materials processed under mesophilic temperature conditions, the time during which the largest part of biogas is released is approximately:

• Cattle liquid manure: 10 -15 days;
• Liquid pig manure: 9 -12 days;
• Liquid chicken manure: 10-15 days;
• Manure mixed with vegetable waste: 40 - 80 days.

Acid-base balance pH

Methane-producing bacteria are best adapted to live in neutral or slightly alkaline conditions. In the process of methane fermentation, the second stage of biogas production is the active phase of acidic bacteria. At this time, the pH level decreases, that is, the environment becomes more acidic.

However, during the normal course of the process, the vital activity of different groups of bacteria in the reactor is equally efficient and acids are processed by methane bacteria. The optimum pH value varies depending on the raw material from 6,5 to 8,5.

You can measure the level of acid-base balance using litmus paper. The values ​​of the acid-base balance will correspond to the color acquired by the paper when it is immersed in the fermentable raw material.

The ratio of carbon and nitrogen

One of the most important factors affecting methane fermentation is the ratio of carbon and nitrogen in the feedstock. If the C/N ratio is excessively high, then the lack of nitrogen will serve as a factor limiting the process of methane fermentation. If this ratio is too low, then such a large amount of ammonia is formed that it becomes toxic to bacteria.

Microorganisms need both nitrogen and carbon to assimilate into their cellular structure. Various experiments have shown that the yield of biogas is greatest at a carbon to nitrogen ratio of 10 to 20, where the optimum varies depending on the type of feedstock. In order to achieve high biogas production, mixing of raw materials is practiced to achieve an optimal C/N ratio.

Table 2. Nitrogen ratio and carbon-to-nitrogen ratio for organic matter

Selecting the right raw material moisture

Unhindered metabolism in the raw material is a prerequisite for high bacterial activity. This is only possible if the viscosity of the raw material allows the free movement of bacteria and gas bubbles between the liquid and the solids it contains. There are various solid particles in agricultural waste.

Solids and dry matter in raw materials

Solid particles, such as: sand, clay, etc., cause the formation of sediment. Lighter materials rise to the surface of the raw material and form a crust on its surface. This leads to a reduction in gas production. Therefore, it is recommended to carefully grind plant residues (straw, leftovers, etc.) before loading into the reactor, and strive for the absence of solids in the raw material.

The dry matter content is determined by the moisture content of the manure. At a moisture content of 70%, the raw material contains 30% solids. Approximate values ​​for the moisture content of manure and excrement (manure and urine) for various animal species are given in Table 4.

Table 3. Amount and humidity of manure and excrement per 1 animal

Humidity of the raw materials loaded into the plant reactor must be at least 85% in winter and 92% in summer. To achieve the correct moisture content of raw materials, manure is usually diluted with hot water in an amount determined by the formula: RH = LF ((B2 - B1): (100 - B2)), where H is the amount of loaded manure, B1 is the initial moisture content of manure, B2 is the required moisture content of raw materials, RH - the amount of water in liters. The table shows the required amount of water to dilute 100 kg of manure to 85% and 92% moisture.

Table 4. The amount of water to achieve the required moisture per 100 kg of manure

Regular mixing

For the efficient operation of the biogas plant and maintaining the stability of the process of fermentation of raw materials inside the reactor, periodic mixing is necessary. The main purposes of mixing are:

• release of produced biogas;
• mixing of fresh substrate and bacterial population (grafting);
• preventing the formation of a crust and sediment;
• prevention of areas of different temperatures inside the reactor;
• ensuring an even distribution of the bacterial population;
• preventing the formation of voids and accumulations that reduce the effective area of ​​the reactor.

When choosing the appropriate method and mixing method, it must be taken into account that the fermentation process is a symbiosis between different strains of bacteria, that is, bacteria of one species can feed another species. When a community breaks up, the fermentation process will be unproductive until a new community of bacteria is formed. Therefore, too frequent or prolonged and intense mixing is harmful. It is recommended to slowly mix the raw materials every 4 - 6 hours.

Process inhibitors

The fermented organic mass should not contain substances (antibiotics, solvents, etc.) that adversely affect the vital activity of microorganisms. Some inorganic substances do not contribute to the "work" of microorganisms, therefore, for example, it is impossible to use water left after washing clothes with synthetic detergents to dilute manure.

Even if toxic materials are not used for biogas production, too high a concentration of individual substances or table salt can retard the growth of bacteria and therefore the production of biogas. The upper limit of some of the most common inorganic substances is given in Table 5.

Table 5. Delay limits for common inorganic inhibitors

Types of raw materials

Cattle manure

Cattle manure is the most suitable raw material for processing in biogas plants, since methane-producing bacteria are already present in the stomach of cattle. The homogeneity of cattle manure allows us to recommend it for use in continuous digestion plants.

Usually, fresh manure is mixed with water and undigested straw is selected from it to prevent sediment and crusting. Cattle urine significantly increases the amount of biogas produced, so it is recommended to build farms with a concrete floor and direct drainage of excrement into a mixing tank.

Pig manure

When keeping pigs in pens and stalls without a paved surface (concrete, wood, etc.), only manure can be used. It must be diluted with water to achieve the correct consistency for processing. The manure diluted with water must settle in a tank so that the sand and small stones present in the manure settle and do not get into the reactor. Otherwise, sand and earth entering the reactor will accumulate at the bottom of the reactor and require frequent cleaning. As in the case of cattle manure, it is recommended to build farms with concrete floors and direct discharge of excrement into a container for mixing raw materials.

Sheep and goat manure

For sheep and goats kept without pavement, the situation is similar to that described for pig manure. Since a goat farm is practically the only place to collect enough manure, and even then only on the condition of straw bedding, the feedstock for a biogas plant is mainly a mixture of manure and straw. Most systems processing such raw materials operate in a batch mode, in which a mixture of manure, straw and water is loaded without prior preparation and remains in the reactor for a longer period than pure manure.

Fig.9. Keeping pigs on a farm with a concrete floor. Photo: Vedenev A.G., OF "Fluid"

Chicken droppings

For the processing of chicken manure, cage keeping of birds or the installation of a perch over a limited area suitable for collecting manure is recommended. In the case of floor keeping of birds, the proportion of sand, sawdust, straw in the litter will be too high. It is necessary to take into account possible problems and clean the reactor more often than when working with other types of raw materials.

Chicken manure is well combined with cattle manure and can be processed along with it. When using pure bird droppings as raw material, there is a risk of high ammonia concentrations. This can lead to low plant efficiency.

Feces

If the faeces are processed in biogas plants, the toilets should be designed so that the faeces are washed away with a small amount of water. It must be ensured that water from other sources does not enter the toilet, and the amount of flushing water should be limited to 0.S - 1 liter of water to prevent excessive dilution of the raw materials.

Fig.10. Combined processing of faeces in a biogas plant c. Belovodskoe. Photo: Vedenev A.G., OF "Fluid"

Gas output and methane content

The gas yield is usually calculated in liters or cubic meters per kilogram of dry matter contained in the manure. The table shows the values ​​of the biogas yield per kilogram of dry matter for different types of raw materials after 10-20 days of fermentation when the plant is operating in mesophilic mode.

To determine the yield of biogas from fresh feedstock using the table, the moisture content of the fresh feedstock must first be determined. To do this, you can dry a kilogram of fresh manure and weigh the dry residue. Humidity of manure in percent can be calculated by the formula: (1 - weight of dried manure) × 100%.

Table 6. Biogas yield and methane content in it when using different types of raw materials

Calculate how much fresh manure with a certain moisture content will correspond to 1 kg of dry matter as follows: subtract the moisture content of the manure in percent from 100, and then divide 100 by this value: 100: (100% - moisture in%).

Example 1: if you determine that the moisture content of the cattle manure used as raw material is 85%, then 1 kilogram of dry matter will correspond to 100:(100 - 85) = about 6,6 kilograms of fresh manure. This means that from 6,6 kilograms of fresh manure we get 0,2S0 - 0,320 m3 of biogas, and from 1 kilogram of fresh cattle manure we can get 6,6 times less: 0,037 - 0,048 m3 of biogas.

Example 2: You have determined the moisture content of pig manure - 80%, so 1 kilogram of dry matter will be equal to 5 kilograms of fresh pig manure. From the table we know that 1 kilogram of dry matter (or 5 kg of fresh pig manure) releases 0,340 - 0.S80 m of biogas. This means that 1 kilogram of fresh pig manure releases 0,068 - 0,116 m3 of biogas.

Approximate values

If the weight of daily fresh manure is known, then the daily yield of biogas in the conditions of Kyrgyzstan will be approximately as follows:

• 1 ton of cattle manure 25-30 m3 of biogas;
• 1 ton of pig manure 50 - 70 m3 of biogas;
• 1 ton of bird droppings 50 - 60 m3 of biogas.

It must be remembered that approximate values ​​​​are given for finished raw materials with a moisture content of 85% - 92%.

Biogas weight

The volumetric weight of biogas is 1,2 kg per 1 m3, therefore, when calculating the amount of fertilizer received, it must be subtracted from the amount of processed raw materials.

For an average daily load of 55 kg of raw materials from one head of cattle and a daily biogas output of 1,5 - 2,0 m3 per head of livestock, the mass of raw materials will decrease by 4 - 5% during processing in a biogas plant.

Peel problem

If a high volume of gas is observed, but it is not combustible enough, this often means that foam or a crust has formed on the surface of the feed in the reactor. If the gas pressure is very low, this may also mean that a crust has formed blocking the gas pipe. It is necessary to remove the crust from the surface of the raw material in the reactor.

Removing the crust

A feature of the crust that forms on the surface of the feedstock in the reactor of a biogas plant is that it is not brittle, but viscous and can become very hard within a short period of time. To destroy it, you need to keep it moist. That is, the crust can be poured over with water or lowered into the raw material.

Sorting of raw materials

Straw, grass, grass stalks and even simply dried manure float to the surface of the raw material, while dry and mineral substances settle at the bottom of the reactor and over time can close the discharge opening or reduce the working area of ​​the reactor. With properly prepared raw materials with a not too high water content, this problem does not arise.

Finished raw material

When using fresh cattle manure, there is no crusting problem. Problems arise when solid and undecomposed organic substances are present in the raw material. Before building the plant, animal feed and manure must be checked for the possibility of processing in the reactor. It may be necessary to carefully grind the feed and in this case it is better to calculate the additional costs in advance. The problem of solids content in raw materials is much more serious for pig manure and poultry manure. Sand pecked by chickens and feathers in droppings make bird droppings a difficult raw material.

Composition of raw materials

Studies of the chemical composition of raw materials before processing in biogas plants were carried out by scientists from foreign countries and Kyrgyzstan.

Table 7. Composition of raw materials before processing in a biogas plant

Viscosity

The viscosity of the raw material during processing is noticeably reduced, since the amount of solid matter (straw, etc.) is reduced by 50% by fermentation under stable conditions.

Smell

Biofertilizer has a much less intense smell than the smell of the raw materials used (manure, urine). With sufficient fermentation time, almost all odorous substances are completely processed.

Nutrients

The nutritional properties of biofertilizer are determined by the amount of organic substances and chemical elements that it contains. All nutrients for plants, such as nitrogen, phosphorus, potassium and magnesia, as well as trace elements and vitamins necessary for plant growth, are stored in biofertilizer. The ratio of carbon and nitrogen (about 1:15) has a favorable effect on soil quality. Table 8 shows the approximate nutrient content of a biofertilizer.

Table 8. The content of elements in biofertilizer (grams per kg of dry matter)

Phosphate and potassium

The content of phosphate (a form of phosphorus directly absorbed by plants) does not change during the fermentation of raw materials. In this form, about 50% of the total phosphorus content can be absorbed by plants. Fermentation does not affect the content of potassium, from 75 to 100% of which can be absorbed by plants.

Nitrogen

Unlike phosphate and potassium, some of the nitrogen changes during fermentation. About 75% of the nitrogen contained in fresh manure becomes part of organic macromolecules, the remaining 25% is in mineral form. After processing in a biogas plant, about 50% of the nitrogen in the biofertilizer is in organic form, and 50% in mineral form. Mineral nitrogen can be directly taken up by plants, while organic nitrogen must first be mineralized by soil microorganisms.

Authors: Vedenev A.G., Vedeneva T.A.

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