3MW cow manure straw biogas power generation project

Project Overview      

ž   Project Name: A Biogas Power Generation Project in Africa

ž   Project Location: South Africa

ž   Project Scale

�?span style="font:9px 'Times New Roman'">   Using cow manure and straw as the main raw materials, with the cow manure coming from 12,000 dairy cows (the material quantity is estimated based on the data of domestic farms of the same scale), and the straw mainly being fresh corn straw and sugarcane straw (with a solid content TS of 23 - 26%). The project scale is as follows:

ž   Project Objectives

�?span style="font:9px 'Times New Roman'">   After the project is completed, it will handle a total of approximately 1680 tons of cow manure and straw per day, with an estimated daily biogas production of 43,400 cubic meters. The biogas will be purified by desulfurization and then all used for power generation. The waste heat from power generation can be recycled to heat the anaerobic system. The biogas residue and biogas slurry produced after anaerobic fermentation and solid - liquid separation will be used for returning to the field.

•     The main newly constructed facilities of this project include: pretreatment system (including silos, crushing devices, grit chambers, homogenization tanks, etc.), CSTR anaerobic system (with an effective volume of 8000 m³ × 6 units), desulfurization and purification system (including biological desulfurization and dry desulfurization, with a treatment capacity of 900 m³/h × 2 sets), biogas storage system (including double - membrane gas holders with an effective volume of 5000 m³ × 1 set), power generation system (with a total of 4MW, including waste heat utilization), flare system, and supporting equipment and facilities.

•     Main Technical Parameters

•     Terminal Product Output

Process Design

•     Process Flow: The treatment process of this project is divided into pretreatment unit, anaerobic fermentation unit, solid - liquid separation unit, biogas purification unit, and biogas utilization unit.

�?span style="font:9px 'Times New Roman'">   Pretreatment Unit: The cow manure is designed to be transported to the septic tank by manure truck or pump (raw material transportation is not included in this plan). The manure overflows into the grit chamber to remove most of the gravel and impurities, and then enters the homogenization tank. The materials are evenly mixed with the crushed straw in the homogenization tank and pre - heated, and then pumped to the anaerobic fermentation tank by a screw pump.

�?span style="font:9px 'Times New Roman'">   Anaerobic Fermentation Unit: The materials enter the anaerobic tank, which is equipped with a stirring and heating device. The fermentation liquid produced by fermentation enters the solid - liquid separation unit; the biogas produced by fermentation enters the biogas purification unit. The heat source for heating in this project mainly comes from the waste heat of power generation, and the hot water circulating pump is used to supply heat to the system structures.

�?span style="font:9px 'Times New Roman'">   Solid - Liquid Separation Unit: All the fermentation liquid after anaerobic fermentation enters the solid - liquid separation system. The biogas residue produced by solid - liquid separation can be used as solid fertilizer and returned to the field; the separated biogas slurry is stored in the biogas slurry temporary storage tank and used as liquid fertilizer.

�?span style="font:9px 'Times New Roman'">   Biogas Purification and Storage Unit: The biogas produced by anaerobic fermentation is purified by desulfurization and dehydration and then stored in the gas holder for later use of biogas.

�?span style="font:9px 'Times New Roman'">   Biogas Utilization Unit: The purified biogas is used for power generation to supply the equipment of this project for self - use, and the surplus power after self - use is connected to the grid for sale.

•     Material Balance: (See the material balance diagram for detailed input, output and conversion of raw materials in each process unit.)

•     Heat Balance

Calculation Basis: Most of South Africa has a warm temperate climate, with an average annual temperature of 10 - 24 °C. The historical monthly average temperature data are as follows:

�?span style="font:9px 'Times New Roman'">   Heat Balance Calculation: Combined with the historical temperature data of the project area, the material temperature of this project is calculated based on 13.7 °C in winter, 18.1 °C in spring and autumn, 21.5 °C in summer, and 17.8 °C on average. The heat balance calculations for the average year and each season are as follows, where the material quantity in winter and summer fluctuates due to the change of spraying water volume:

�?span style="font:9px 'Times New Roman'">   Introduction to Main Process Units

•     Pretreatment Unit

�?span style="font:9px 'Times New Roman'">   Grille: A rotary mechanical grille is installed at the front end of the homogenization tank. The grille is used to intercept and remove weeds, hairs and large impurities. When the transmission system drives the sprocket to rotate at a constant speed and in a fixed direction, the entire rake chain moves from top to bottom and carries the fixed impurities to be separated from the liquid. The fluid passes through the gaps of the rake teeth. The whole working state is continuous. Due to the special structure of the rake teeth, when the rake teeth carry the impurities to the upper end and move in the opposite direction, a relative self - cleaning movement is generated between the front and rear rake teeth, which promotes the impurities to fall off due to gravity. At the same time, the rear cleaning device is set to clean the impurities stuck on the rake teeth to ensure that each row of rake teeth can be thoroughly cleaned when moving to this position.

�?span style="font:9px 'Times New Roman'">   Spiral Sand Removal Equipment

�?span style="font:9px 'Times New Roman'">   Equipment Description: After the mechanical grille removes impurities, the remaining impurities in the manure are mostly sandy impurities. In the grit chamber, the manure is evenly flowed to the periphery of the tank by the central drive scraper. Most of the gravel wrapped in the manure is completely separated from the raw manure. As the flow velocity decreases, most of the coarse sand and a small amount of fine sand mixed in the material are separated and settled at the bottom of the tank. The bottom scraper scrapes the settled gravel and other impurities to the inlet of the spiral sand remover, and the impurities are output from the outlet after being pushed by the spiral.

�?span style="font:9px 'Times New Roman'">   Equipment Advantages: Through the multi - stage impurity removal device of grille impurity removal + spiral sand removal, 90% of the impurities in the manure can be removed, thus avoiding the impact on the subsequent anaerobic fermentation equipment and pipelines and ensuring the normal and stable operation of the system. The impurity removal system equipment can control the cost while ensuring the ease of use and reliability of the equipment.

�?span style="font:9px 'Times New Roman'">   Straw Crushing Equipment: The working principle is as follows: rotor centrifugal crushing. The feed is crushed and mashed into a fibrous shape by impact and shear forces. With the help of the biological material crusher, the gas production can be accelerated and the whole process can be stabilized. In addition, the variety of raw materials for treatment is increased, and the economic efficiency of the biogas production station is significantly improved. The residence time of the substrate in the biological material crusher is very short, so almost all the energy used is converted into crushing kinetic energy instead of heat energy, and the stirring cost is also reduced.

•       Anaerobic Fermentation Unit

�?span style="font:9px 'Times New Roman'">   CSTR Anaerobic Fermentation Tank: The anaerobic fermentation of the manure materials in this project adopts a continuous stirred tank reactor (CSTR). This reactor is a reactor in which a stirring device is installed in a conventional digester to make the fermentation raw materials and microorganisms in a completely mixed state. Its efficiency is significantly higher than that of the conventional digester and is currently a widely used reactor in large and medium - sized biogas projects. The fermentation raw materials are added regularly or continuously into the digestion tank. After mixing and contacting with the original anaerobic activated sludge in the tank, the organic matter in the raw materials is converted into biogas through the adsorption, absorption and degradation of anaerobic microorganisms. The produced biogas is discharged from the top of the tank, and the fermentation liquid is discharged from the upper part of the reactor. This project adopts a top - entry mixer as the stirring device of the reactor. The motor and reducer are fixed on the top of the CSTR anaerobic tank, and the stirring shaft extends vertically into the tank. The blades rotate around the long axis. The mixing of materials is realized by the stirring action of the mixer to avoid sedimentation and stratification.

�?span style="font:9px 'Times New Roman'">   Equipment Advantages: The tank feeding design is simple and does not require special design of the feeding water distribution device; the biogas fermentation rate is greatly improved by using the stirring technology in the biogas fermentation tank; the anaerobic fermentation tank is equipped with a heating and insulation device to maintain the tank at a suitable temperature; the tank manufacturing process is mature, and the design of the anaerobic tank meets the relevant process and use requirements.

�?span style="font:9px 'Times New Roman'">   CSTR High - Efficiency and Energy - Saving Anaerobic Tank Top Mixer: Each anaerobic fermentation tank in this project is equipped with a high - efficiency and energy - saving tank top center mixer, which makes the feed evenly distributed and fully contacts with the anaerobic microorganisms, and makes the temperature of the material liquid in the anaerobic tank uniform, which is conducive to improving the gas production rate. The high - efficiency and energy - saving anaerobic tank mixer adopts Danish technology, with a low installed power per unit fermentation tank volume, saving more than 50% of energy compared with traditional mechanical stirring and jet stirring. The high - efficiency and energy - saving anaerobic tank mixer adopts multi - layer blades to fully mix and stir the materials, ensuring sufficient mass transfer and heat transfer in the tank, and avoiding the occurrence of fermentation dead zones and local acidification in the tank. The mixer has a shell - breaking function to prevent the materials and scum on the top of the anaerobic tank from crusting while ensuring uniform stirring in the tank.

•     Biogas Purification Unit

�?span style="font:9px 'Times New Roman'">   Biological Desulfurization Process Principle: The biological desulfurization process is a new technology that uses desulfurization microorganisms to remove hydrogen sulfide in biogas. Its principle is to rely on Thiobacillus and Thiothrix to absorb hydrogen sulfide and convert it into sulfuric acid during metabolism. Using this process for biogas desulfurization can achieve a hydrogen sulfide removal efficiency of more than 97%. Its process flow and principle are as follows:

�?span style="font:9px 'Times New Roman'">   Absorption and Conversion of Hydrogen Sulfide: The upper part of the biological desulfurization tower is a spraying device, and the middle part is a filler layer for the attachment and growth of desulfurization microorganisms, forming a biological "filter screen". The nutrient solution is continuously sprayed in a mist form from the top of the biological scrubbing tower by a circulating pump to keep the filler wet and supplement the nutrients required for the growth and reproduction of microorganisms. The biogas enters from the lower part of the biological desulfurization tower and is supplied with an appropriate amount of oxygen. The hydrogen sulfide in the biogas is fully absorbed by the circulating liquid and then converted into sulfur element by the desulfurization microorganisms attached to the filler in the tower. Under suitable conditions, it is further oxidized into sulfuric acid. The purified biogas is output from the outlet at the top of the tower and enters the next section. The main chemical reaction equations are as follows:

�?span style="font:9px 'Times New Roman'">   Regeneration of Absorption Liquid: Under the conditions of sufficient nutrients and suitable environment, the desulfurization microorganisms can reach the required population quantity for biological desulfurization in a relatively short time. Therefore, it is necessary to regularly provide the required nutrient elements for the desulfurization microorganisms. Under normal circumstances, the anaerobically treated and filtered biogas slurry can meet the growth, reproduction and replacement needs of the desulfurization microorganisms. It is also possible to choose to configure the nutrient solution with softened water to provide nutrients for the desulfurization bacteria microorganisms. To avoid the blockage of the filler in the biological desulfurization tower, all the water used in the system needs to be softened water.

�?span style="font:9px 'Times New Roman'">   Configuration of Nutrient Solution: Due to the generation of sulfur element, sulfurous acid and dilute sulfuric acid and other substances in the reaction process of hydrogen sulfide, the pH of the circulating liquid changes. Therefore, it is necessary to regularly discharge a small amount of waste liquid to remove the elemental sulfur in the circulating liquid and stabilize the pH value of the circulating liquid to create a suitable environment for the desulfurization microorganisms.

�?span style="font:9px 'Times New Roman'">   Process Characteristics: The biological desulfurization has the following process characteristics: high desulfurization efficiency, generally the hydrogen sulfide removal rate can reach 97%; compared with other desulfurization methods, the investment is lower; the desulfurization equipment is compact and has a small floor area; basically no pollutants are generated, and there is no need to treat solid sulfur element; it can be automatically controlled and operated.

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