The process of biogas recycling generally includes several steps: raw material collection and pretreatment, anaerobic fermentation to produce gas, biogas purification and purification, biogas storage, and biogas utilization. The following is an introduction for you:
Raw material collection and pretreatment
Raw material collection
The raw materials suitable for producing biogas come from a wide range of sources, including agricultural waste (such as straw, livestock manure), industrial organic wastewater, urban domestic sewage, and garbage. A stable raw material collection system needs to be established based on the local actual situation. For example, in rural areas, centralized collection points can be established to collect animal manure and straw; In the city, cooperate with sewage treatment plants and garbage disposal sites to obtain corresponding raw materials.
Raw material pretreatment
Crushing: For coarse raw materials such as straw, crushing treatment is required to increase the contact area between the raw materials and microorganisms, and improve fermentation efficiency. Generally, straw is crushed to a length of 2-5 centimeters.
Adjusting the carbon nitrogen ratio: The appropriate carbon nitrogen ratio (C/N) is the key to biogas fermentation, usually controlled at 20-30:1. The carbon nitrogen ratio can be adjusted by mixing different raw materials. For example, if the carbon nitrogen ratio of animal manure is low and that of straw is high, the two can be mixed in a certain proportion.
Adjust moisture content: The moisture content of raw materials is generally controlled between 80% and 90%. If the moisture content is too high, it can be reduced by drying or other methods; If the moisture content is too low, an appropriate amount of water needs to be added.
Anaerobic fermentation produces gas
Selection of Fermentation Equipment
Common anaerobic fermentation devices include traditional hydraulic digesters, upflow anaerobic sludge beds (UASBs), and fully mixed anaerobic reactors (CSTRs). Suitable fermentation equipment should be selected based on factors such as raw material characteristics, processing scale, and local climate conditions. For example, small-scale rural biogas projects can use hydraulic biogas digesters; Large scale industrial organic wastewater treatment often uses UASB or CSTR reactors.
Vaccination and initiation
Add inoculants containing a large amount of anaerobic microorganisms, such as sludge and sludge from old biogas digesters, to the fermentation device to accelerate the fermentation start-up speed. The vaccination amount is generally 10% -30% of the raw material volume. During the start-up process, it is necessary to control the fermentation temperature, pH value, and other parameters to allow anaerobic microorganisms to gradually adapt to the new environment and begin producing gas.
Fermentation process control
Temperature control: Anaerobic fermentation can be divided into room temperature fermentation (10-30 ℃), medium temperature fermentation (around 35 ℃), and high temperature fermentation (around 55 ℃). Medium temperature fermentation and high-temperature fermentation have higher gas production efficiency, but require a certain amount of energy to maintain temperature. In practical applications, the appropriate fermentation temperature can be selected based on local climate conditions and energy supply.
PH control: The suitable pH range for anaerobic fermentation is 6.5-7.5. If the pH value is too high or too low, it will affect the activity of microorganisms and lead to a decrease in gas production. The stability of pH value can be maintained by adding acid-base regulators.
Stirring: Regularly stirring the fermentation raw materials can ensure full contact between the raw materials and microorganisms, promote material transfer and gas production. The stirring methods include mechanical stirring, gas stirring, and liquid stirring.
Biogas purification and purification
desulphurization
Biogas contains a certain amount of hydrogen sulfide (H ₂ S), which is corrosive and toxic, and can cause damage to equipment and pipelines, while also affecting the combustion performance of biogas. The commonly used desulfurization methods include dry desulfurization and wet desulfurization. Dry desulfurization uses desulfurizers such as iron oxide, which is suitable for small-scale biogas projects; Wet desulfurization uses solutions such as sodium hydroxide and sodium carbonate to absorb hydrogen sulfide, which is suitable for large-scale biogas projects.
Dehydration
Biogas contains a large amount of water vapor, which can reduce the calorific value of biogas and cause pipeline freezing and blockage in low temperature environments. Dehydration treatment can be carried out using methods such as condensation and adsorption. The condensation method is to lower the temperature of biogas to condense and precipitate water vapor; Adsorption method is the use of desiccants (such as silica gel, molecular sieves, etc.) to adsorb water vapor.
Decarbonization
To improve the calorific value and quality of biogas, it is necessary to remove carbon dioxide (CO ₂) from it. The commonly used decarbonization methods include absorption, pressure swing adsorption (PSA), and membrane separation. The absorption method uses solutions such as sodium hydroxide and ethanolamine to absorb carbon dioxide; The PSA method utilizes the differential adsorption selectivity of adsorbents to separate carbon dioxide and methane; The membrane separation method utilizes the different permeation rates of gases in the membrane to achieve the separation of carbon dioxide and methane.
Biogas storage
Low voltage storage
Low voltage storage generally uses wet or dry gas storage tanks. Wet gas storage cabinets use water seals to store gas, with a simple structure and low cost, but they occupy a large area and have high maintenance costs; Dry type gas storage cabinets use rubber membranes or pistons to store gas, occupying a small area and having good sealing performance, but the cost is relatively high.
High voltage storage
For biogas that requires long-distance transportation or as vehicle fuel, high-pressure storage can be used. Compress the purified biogas to 20-25MPa and store it in high-pressure gas cylinders or high-pressure gas storage tanks. High voltage storage requires specialized compression equipment and safety devices.
Biogas utilization
electricity generation
Using biogas as fuel to drive a generator can meet one's own electricity needs or be integrated into the power grid. The biogas power generation system mainly consists of biogas engines, generators, waste heat recovery devices, etc. The waste heat generated during the power generation process can be used to heat fermentation materials or meet other heat needs, improving energy utilization efficiency.
Heating supply
Biogas can be directly used for boiler combustion to produce steam or hot water, providing thermal energy for industrial production, residential life, etc. Compared with traditional coal-fired and oil fired boilers, biogas boilers have the advantages of being clean, environmentally friendly, and having low operating costs.
As a vehicle fuel
After purification and purification, the main component of biogas, methane content, can reach over 97%, which is similar in composition and properties to natural gas. It can be used as a substitute for gasoline and diesel as a vehicle fuel. Vehicle biogas needs to be stored in high-pressure gas cylinders and equipped with corresponding refueling stations and vehicle modification equipment.
Production of chemical products
Methane in biogas can be used as a chemical raw material for the production of chemical products such as methanol, formaldehyde, and synthetic ammonia. By using chemical synthesis technology, biogas can be converted into high value-added chemical products, which can achieve efficient utilization and value-added of biogas.
