1, what is biogas?
Biogas fermentation, also known as anaerobic digestion, refers to the process in which various biogas fermentation microorganisms decompose and transform human and animal manure, straw, sewage and other organic substances under anaerobic conditions (anaerobic) in a sealed biogas digester, and finally produce biogas. In this over-planting, microorganisms are the most active factor. According to their respective nutritional needs, they decompose and transform all kinds of solid or dissolved complex organic matter, and finally produce biogas. Biogas is a combustible mixture. Because this gas was first found in swamps, it is called biogas. Its main components are about 55%-70% methane and about 25%-40% carbon dioxide. In addition, there are a small amount of hydrogen, hydrogen sulfide, carbon monoxide, nitrogen and ammonia.
2, biogas fermentation microorganisms
In the process of biogas fermentation, there are five kinds of microorganisms involved in biogas fermentation: fermenting bacteria, hydrogen-producing acetic acid-producing bacteria, oxygen-consuming acetic acid-producing bacteria and hydrogen-producing methanogenic bacteria. Their functions in the fermentation process and their requirements for living conditions can be divided into the following three stages:
First stage descent: liquefaction
In biogas fermentation, firstly, fermentation bacteria use extracellular enzymes secreted by them, such as cellulase, amylase, protease, lipase, etc., to hydrolyze organic substances in vitro, that is, macromolecular organic substances such as livestock manure, crop straw, and wastewater after bean products processing are decomposed into water-soluble monosaccharides, amino acids, glycerol and fat. This stage is called liquefaction stage.
The second stage: acid production
This stage is the joint action of three flora. Firstly, the small molecular compounds produced in the liquefaction stage are absorbed into cells by fermentation bacteria and decomposed into acetic acid, butyric acid, hydrogen and carbon dioxide, and then propionic acid and butyric acid produced by fermentation bacteria are converted into acetic acid, hydrogen and carbon dioxide which can be utilized by methanogens.
In addition, there are oxygen-consuming acetic acid-producing bacteria, which use oxygen and carbon dioxide to produce acetic acid, and can also metabolize sugars to produce acetic acid. They can convert many organic substances into acetic acid.
Liquefaction stage and acid production stage are a continuous process, which are collectively called methane-free stage. In this process, there are many kinds of methanogenic bacteria, whose main function is to provide nutrition for methanogenic bacteria, create suitable anaerobic conditions for methanogenic bacteria and eliminate some poisons.
The third stage: methane production
At this stage, methanogens can be divided into two types: hydrogen-eating methanogens and acetic acid methanogens. More than 70 methanogenic bacteria have been studied, which use the above three non-methanogenic bacteria to decompose and transform small molecular compounds such as formic acid, acetic acid, hydrogen and carbon dioxide to produce methane.
① Growth is very slow. For example, when methanococcus grows on acetic acid, its proliferation time is 1-2 days, and the doubling time of methane mycelium is 4-9 days. (2) Strictly anaerobic, very sensitive to oxygen and oxidant, unable to survive or die in the air; ③ Only a few simple compounds can be used as nutrition; (4) They are required to be in neutral alkaline and suitable temperature and environmental conditions; ⑤ The main end products of metabolic activities are methane and carbon dioxide.
3. Biogas fermentation conditions
Biogas fermentation is completed by many kinds of bacteria, and they need certain conditions for biological activities in the process of metabolism, growth and reproduction in biogas digesters. Only by artificially creating suitable production conditions for them can a large number of microorganisms multiply rapidly and accelerate the decomposition of organic matter in biogas digesters. On the other hand, certain conditions are needed to control the normal operation of the biogas digester fermentation process. Therefore, only by meeting the growth conditions of microorganisms and the normal operation conditions of biogas digesters can we achieve the effect of high gas production rate and more organic biogas manure.
To sum up, the basic conditions of artificial gas production are: biogas strain, fermentation principle, fermentation concentration, PH value, strict anaerobic environment and suitable temperature. One of the conditions is not suitable for biogas bacteria and cannot produce biogas.
(1) Biogas production must use biogas bacteria, which is the same as yeast for fermentation. If there are no biogas bacteria, the organic matter in the biogas digester itself will not be converted into biogas, so there must be enough inoculum containing excellent biogas strains at the beginning of biogas fermentation, which is an important condition for biogas production.
Inoculants containing excellent biogas strains in rural areas generally exist in the sludge at the bottom of cesspits, sewage sludge, biogas fermentation residue water, biogas sludge, and sludge in the drainage ditch of bean products workshops. These sludge containing a large number of biogas fermentation bacteria are called inoculants. The operation process of adding inoculum in biogas fermentation is called inoculation. If the new biogas digester is charged for the first time, it is often difficult to produce biogas or the yield is not high, the methane content is low, and it cannot be burned without adding enough inoculum containing biogas bacteria. In addition, adding an appropriate amount of inoculum can prevent fermentation from being blocked due to excessive acid production in the early stage of fermentation.
(2) Sufficient fermentation raw materials Biogas fermentation raw materials are the material basis for producing biogas, and also the nutrient sub-source for biogas to survive through fermenting bacteria, because in the normal growth and reproduction process of biogas bacteria in biogas digesters, sufficient nutrients, such as water, carbon, nitrogen, inorganic salts and auxin, must be absorbed from the fermentation raw materials for life activities, breeding bacteria and producing biogas.
Carbohydrate in organic matter, such as cellulose and starch in straw, is the carbon nutrition of bacteria, and organic nitrogen in organic matter, such as nitrogen in animal feces and urine, is the nitrogen nutrition of bacteria. When organic matter is decomposed by bacteria, some carbon and nitrogen of organic matter are assimilated into bacterial cells to form other new substances, and the other part is decomposed into simple organic matter by acid-producing bacteria, and methane is produced by methanogenic bacteria. Therefore, when biogas is fermented, the raw materials should not only be sufficient, but also be properly matched. Maintain a certain ratio of carbon to nitrogen, so that the lack of carbon and nitrogen nutrition will not affect the production of biogas and the normal reproduction of bacteria.
(3) Concentration of fermentation raw materials The feed liquid in the biogas digester needs to maintain a certain concentration in the fermentation process to produce gas normally. If the water content of the feed liquid is too low, the fermentation raw materials are too high, and the concentration of the fermentation liquid is too high, methanogenic bacteria can't eat so much, which is easy to cause a large accumulation of organic acids and hinder fermentation; If there is too much water, the concentration of fermentation broth is too dilute, and the organic matter content is low, the gas production will be small. Therefore, the fermentation broth of biogas digester must maintain a certain concentration. According to many years' practice, the concentration of fermentation broth in rural biogas digesters is generally 6%- 10%. In this range, the initial start-up concentration of biogas is low to promote start-up. In summer and early autumn, the pool temperature is high, the raw materials decompose quickly, and the concentration can be appropriately lower; In winter and early spring, the concentration of fermentation broth with low pool temperature and slow decomposition of raw materials should be kept at 10%.
(4) The proper PH is expressed by PH. Put a pH test paper into the stirred fermentation broth, take it out immediately, and compare it with the standard colorimetric card attached to the pH test paper, so that the pH value of the fermentation broth can be known from the color change.
The optimum PH of biogas fermentation bacteria is 6.8-7.5, and the gas production below 6.4 and above 7.6 is inhibited. If the PH is lower than 5.5, it is a sign of acidification of feed liquid, and the activity of methanogenic bacteria is completely inhibited. For example, when biogas is initially started, the feeding concentration is too high, the number of methanogenic bacteria in the inoculum is insufficient, or the concentration of fermentation broth is too high due to the addition of a large amount of chicken manure and potato residue at one time, which will lead to the unbalanced speed of acid production and methane production, leading to the accumulation of volatile acids (acetic acid, propionic acid and butyric acid) and the decrease of pH. This is the main reason for the failure of biogas digester start-up or abnormal operation.
In the process of biogas fermentation, the change law of PH value is generally as follows: in the early stage of fermentation, due to the rapid activity of acid-producing bacteria, a large number of organic acids are produced, which makes the PH value decrease; However, with the continuous fermentation, on the one hand, ammonia produced by ammonifying bacteria neutralized some organic acids; On the other hand, methanogenic bacteria use organic acids to convert into methyl bromide, so that the PH value returns to normal. If this cycle continues, the PH value in the biogas digester will be kept in the range of 7.0-7.5, and the fermentation will run normally. Therefore, when the biogas digester liquid is fermented, it can be fermented normally without adjustment as long as a certain concentration, inoculation amount and suitable temperature are maintained.
(5) Anaerobic decomposing bacteria and methanogenic bacteria play an important role in biogas fermentation under strict anaerobic environment. They are afraid of oxygen and will die after being exposed to the air for a few seconds, which means that the oxygen in the air is toxic to them. Therefore, strict anaerobic environment is one of the most important conditions for biogas fermentation. According to the characteristics that biogas bacteria are afraid of air, we adopt a double sealed tank with resin and GRC, and the water-gas sealing performance can completely meet the requirements of biogas fermentation. In the process of use, as long as there is no hard object impact and special accident, there is no air leakage problem for a long time.
(6) Appropriate temperature The temperature of the fermentation broth in the biogas digester has a great influence on the gas production, because in the most appropriate temperature range, the higher the temperature, the faster the growth and reproduction of biogas bacteria, and the more gas production. If the temperature is not suitable, biogas bacteria grow slowly and produce little or no gas. Therefore, temperature is an important condition for biogas production.
How high is the temperature suitable? Generally speaking, biogas bacteria can ferment at 8-60℃. Biogas fermentation is divided into three fermentation zones, namely normal temperature fermentation zone 10-26℃, moderate temperature fermentation zone 28-38℃, and the optimum temperature is 35℃; In the high-temperature fermentation area of 46-60℃, rural biogas fermentation is generally carried out at room temperature due to the limitation of conditions. We installed a solar energy efficiency device in the biogas digester. For example, when the fermentation temperature is low, we can open the heating valve in the solar water heater and introduce hot water into the tubular radiator in the pool to supplement the temperature of the fermentation liquid. Especially in winter, solar synergistic devices will play a great role in biogas fermentation and gas production rate.